[OpenMP][Docs] Updated the OpenMP documentation about building the OpenMP documentation with Sphinx

When I was trying to improve the OpenMP documentation, I found that the information in `OpenMP/docs/README.md` did not contain up-to-date information about how to build the OpenMP documentation with Sphinx. When I ran `make
docs-openmp-html`, the command failed because there were a few syntax errors in `openmp/docs/design/Runtimes.rst`. This commit fixes the syntax errors and updates the documentation on building the OpenMP documentation.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D156470