Javadoc cleanup (#421)

The changes were largely driven by Java 8's enhanced javadoc linting.

* Only use header tags if the previous number has been used in the page (start at h3)
* Remove empty p tags
* Remove improper p end tags
* Fix some dead link references
* Correct unescaped special HTML characters in code examples (less than, greater than, ampersand)
* Fix a couple bad pre tag attributes
* Fix an improperly nested list
* Fill in some missing @param and @return docs
Update api links to include trailing slash (#421)

Java 8 seems to complain about redirects for these URLs
Fix a MethodSelectionException for Java 8 (#421)

Java 8 added a new signature for remove to Map that takes two arguments.
Without the arguments specified, it can't figure out which one, and throws a MethodSelectionException.
Fix a javadoc link