Clarified note on multi-threading in the docs.

Change-Id: Ib36ec71aab02fc66d2a6d0c916ebfa4dd1d0d02f
Added user defined attribute declarations.

This is such that if you mis-spell an attribute, it doesn't get
silently ignored.

Bug: 18294628
Change-Id: I10013f5b2a21048b7daba2e9410678f528e09761
Tested: on Linux.
Added "final" to generated types to block inheritance.

People sometimes accidentally inherit from these types.

Bug: 18224703
Change-Id: Ia09489a834ac4941f9b4a46f240cbdcf456f03a1
Tested: on Windows and Linux.