[OpenMP][doc]Minor doc fixes

In SupportAndFAQ.rst, add blank lines before and after a bullet list and
sublist.  This avoids an "Unepxected indentation" warning.

In Runtimes.rst, adjust the suggestion for setting LIBOMPTARGET_INFO.
The right shifts are not necessary as the bit mask values are already
correct.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D119595