Add a new attribute documentation category for declarations.

Add a new attribute documentation category for declarations.

This moves documentation for some attributes into new categories that are hopefully a bit more clear. In general, "Type" documentation should be for attributes that appertain to types while "Declaration" documentation should be for attributes that appertain to declarations other than functions or variables.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@357585 91177308-0d34-0410-b5e6-96231b3b80d8