Update docstring for FormValue()

[AngelVI13]

Description

Improve docstring for FormValue() to Include the other places that are searched for the provided form value key.

Context:
I was using FormValue() to get the contents of a MultipartForm, however, the request also had some query arguments with the same name. The value I got from FromValue() was the one from the query args and not from the multipart form. This was not expected since the documentation clearly stated that it is getting the value from a MultipartForm. I checked the implementation of FormValue() and saw that it is using fasthttp which explains the order of the search for form value as follows:

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

None of the above. Its just a doc fix.

Checklist:

• For new functionalities I follow the inspiration of the express js framework and built them similar in usage
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - https://github.com/gofiber/docs for https://docs.gofiber.io/
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• If new dependencies exist, I have checked that they are really necessary and agreed with the maintainers/community (we want to have as few dependencies as possible)
• I tried to make my code as fast as possible with as few allocations as possible
• For new code I have written benchmarks so that they can be analyzed and improved

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