[JAVA-1410] Documenting com.mongodb.gridfs

As per the first pass of documenting the legacy API:
 - Adding missing Javadoc for public classes and methods
 - Applied simple standardisation to existing documentation (formatting, capitalisation)
 - Renamed params where appropriate for better readability.
 - `@Override` used to ensure parent documentation applied to child methods, and in some cases child documentation has been removed where it does not add anything to parent docs.

Although existing documentation has been scanned, no effort has been made to ensure its correctness.
Mmm, good point.  There is one, but it's not public.  I'll remove that line from the docs.