Editing

The best training for copy-editing is OJT and mentoring. If possible, you should try to get work at a publishing house. For an entry level position, they usually give you some sort of test to see what aptitude you might have for the work, and then give you training.
However, increasingly the opportunities in this field are freelance, and not full-time permanent positions at publishing houses. Indeed, unless you are in a publishing center such as any or Boston, full-time positions may be almost impossible to get. This creates a chicken and egg problem: One cannot get work without having had experience, but one cannot get experience without having had work.
If you are in this bind, then any kind of certification or training you can get will be helpful. One also cannot stress enough the need to start connecting with authors, editors, and people in the publishing industry, as the vast majority of work in this field is unadvertised.
I also agree with Stephanie Fysh's suggestion about not working for free. While it might be worth the trouble to do one or two small projects at a low price for a friend or relative just to get something to put on your resume, there is a whole underclass of writers and editors who are selling themselves short by offering their services for free or at very low rates all the time. They may think that they are building a career that way, but they are only hurting themselves.
In publishing, a serious client will pay a good wage for your labor. One sees on Internet message boards people complaining about not getting paid for a job or being mistreated by a client, and it is nearly all over jobs paying less than $40. However, people who make a serious career from this work generally get paid much more than this unless it is for a very small job, and they generally get paid on time, because they are working for people who are equally as serious about publishing as they are.

The first question: Should I opt for higher studies or pursue a job after completing a course in non-linear editing? It deserves a yes for a job if you are getting a job because the actual purpose of doing a professional skill oriented course or training is to put it into use and start earning the vocational income.
If the non-linear editing, which is much in use now, is coming your way in the form of a stable job where your skills will further improve, take that as a course of higher studies as well. Getting paid for learning will be a better thing than joining another institute just because the previous place did not teach you enough to get a job.
Why: Video editing has worked for all. Those who know video editing and have the interest to work on the same field, have work. It is just about making a good connection as well as presenting good work, which gets you more work and strengthens your career.

Final Cut Pro is the leading editing skill software which has more demand abroad than in India. There are jobs and good salaries as well but the number is less because some easy softwares are preferred over this here. The scope is good abroad.
Why I mentioned scope is because the reason of doing higher studies or going for a job depends on whether you are interested in going abroad for jobs in FCP. Not just Hollywood but TV Channels and web series world allows good work good jobs.
Hence, my submission is to go for a job because FCP will demand on field practice.
However, if your aforesaid certificate is not able to get you a job and that is an entry level course only, then of course, higher studies will be needed to gain skills.
Job seems a more sensible decision because there will be an opportunity with you to pursue some media related or mass communication degree through distance or part-time.

You may think of the following courses depending on the qualifications you already have:
1. If the current course is done after doing graduation then a Post-Graduation in Mass Communication/Mass Media/Media Production can also be good for you. I shall not hesitate to suggest even a Master Degree in A Language- English, Hindi or the one you see as an additional strength for editing and proofreading works.
2. If one does Certificate in Proofreading and Editing after class 12th, pursue Graduation as a must to do thing. Any Stream you may choose keeping in mind that an Alternative Career has to be met openly. Proofreading and Editing Job can also be a part-time thing so you have enough time to do another degree or Higher Studies.
3. If this Certificate Course is done by someone who is a Master Degree Holder, if not in Language or Social Science, Humanities, do one degree.

Non-linear editing is a specialisation domain, and there may be many things that you will need to keep learning along with the production learning.
The suggestions that I can make are:
1. A degree in a popular language that shall be used in your works and place.
2. Content writing and proof reading.
3. Web development and digital marketing.
4. Music and sound mixing basics/audio production.
5. Advertising and public relations management.
The decision depends on how much time you get free from the work of learning documentary production. When you say that you are learning the production of documentary-I take it as a comprehensive work that is itself a time-taking thing and might not allow you to venture for more.