JSR 292 javadoc needs cleanups and clarifications

The javadoc tool running on the source files for java.lang.invoke turns up a lot of errors and warnings. Some of the errors cause the tool to bail out in some build modes.

Some of the source code examples have naked '<' and '>' characters which need to be replaced by html entities.

Some of the language is imprecise, such as a sentence in Lookup which seems to refer to the return value of a constructor.

Fixing or clarifying these issues does not change types or behavior of the API, with the following minor exceptions:
- (none at present)