FindOpenGL: on macOS, support X11 OpenGL

On macOS, FindOpenGL will not find the OpenGL support for X11 apps, since that uses different library names than the macOS OpenGL frameworks. This MR adds the appropriate names and a bit of explanatory commentary.

Discussion at https://discourse.cmake.org/t/x11-and-opengl-builds-on-macos/9466

Topic-rename: FindOpenGL-macOS-x11

Ooops! Pressed the wrong button and this went out before I'd finished writing the overview.

Summary: on macOS, FindOpenGL will not find the OpenGL support for X11 apps, since that uses different library names than the macOS OpenGL frameworks. This MR adds the appropriate names and a bit of explanatory commentary.

changed the description

changed milestone to %3.29.0

added workflow:in-review label

requested review from @brad.king

added 2 commits

• a13f6242 - FindOpenGL: Clarify cache entry doc strings on macOS
• 1dbfe638 - FindOpenGL: Document how to use macOS X11 OpenGL

Compare with previous version

Do: stage

added workflow:nightly-testing label and removed workflow:in-review label

added 3 commits

• 1dbfe638...a6853135 - 2 commits from branch cmake:master
• 304f8258 - Merge branch cmake:master into macOSx11OpenGL

Compare with previous version

resolved all threads

Do: stage

resolved all threads

Do: merge

mentioned in commit 84a3e839

merged

removed workflow:nightly-testing label

mentioned in issue #6057 (closed)