Update docs for CheckReturnValue

The "How to tell Error Prone which methods to check" section doesn't seem to reflect the changes made in https://github.com/google/error-prone/pull/435.

It also would be better to expand the "Ignored contexts" section to fully reflect the following:

https://github.com/google/error-prone/blob/b1631feee1b2cd2a2bb420a4ed712ebf36920041/check_api/src/main/java/com/google/errorprone/matchers/Matchers.java#L1330-L1353

See https://errorprone.info/bugpattern/CheckReturnValue