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.