docs: clarify the role of compiler-builtin type trait primitives

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).

llvm-svn: 199562