Update the documentation to clarify the intent of clang's built-in type trait
facilities, their relation to user-facing C++ type traits and means to check
for availability.

Also explain that __has_feature() is not currently up to date and should not
generally be used in user code (there's a proposal to provide more consistent
checks via __has_builtin(), see cfe-dev).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199562 91177308-0d34-0410-b5e6-96231b3b80d8