[Clang][OpenMP] Don't overload "extension" in status doc

In Clang's OpenMPSupport.rst, "extension" is currently overloaded to
describe both:

1. Standard OpenMP features that appear only in recent versions of the
   OpenMP spec.
2. Non-standard features supported by Clang.  This usage appears in
   the final table on the page.

Last fall, we discussed this issue in the OpenMP in LLVM call and
agreed it should be corrected.  This patch takes the simple approach
of dropping the word "extension" for all occurrences of the first
usage.  The result seems to read well.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D126549