Update the documentation for attribute feature tests.
Update the documentation for attribute feature tests.

This clarifies that __has_cpp_attribute is no longer always an extension since it's now available in C++2a. Also, Both __has_cpp_attribute and __has_c_attribute can accept attribute scope tokens with alternative spelling (clang vs _Clang and gnu vs __gnu__).

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@347312 91177308-0d34-0410-b5e6-96231b3b80d8

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