[JAVA-1410] Documenting com.mongodb.util

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, and occasionally variables, where appropriate for better readability.
 - @Override used to ensure parent documentation applied to child methods.

Although existing documentation has been scanned, no effort has been made to ensure its correctness.