Documentation Improvements

[rnc]

When working on the upgrade to the latest Unirest I noticed a few small things that would be helpful if they were documented in the user guide. Some of these were talked about in issues, but not added to the guide.

• Missing mapError
• Missing mapBody
• GenericType - while the example shows it being used ( https://kong.github.io/unirest-java/#responses ) it is missing javadoc and a description within the guide.
• Metrics section:
  • Spelling : Context inforation
  • Grammar: In it’s simplest form it might like this --> In it’s simplest form an example may be:

Excellent upgrade, the client code is now much simpler! Thanks!

[ryber]

should be better. the config in the old system was all over the place, so I made it clear that everything is now in config()