libbinder_ndk: document preferred error types

The anonymous enums corresponding to binder_status_t and
binder_exception_t are undocumented, but currently these show up on the
developer site w/ documentation like "Anonymous Enum 19". This is aimed
at fixing that and also explaining which to use when.

Bug: N/A
Test: N/A
Change-Id: I20414858beda591fe9f4f224b7de237bed4c97ac