Update "Using Python on macOS" document for 3.15.0

[ned-deily]

Update usage documentation for macOS for 3.15 to include changes like the default installation of free-threaded support.

Linked PRs

• gh-153872: Update macOS installer ReadMe files for 3.15. #153875
• [3.15] gh-153872: Update macOS installer ReadMe files for 3.15. (GH-153875) #153876
• gh-153872: macOS: free-threading support is installed by default on Python 3.15 #154103
• [3.15] gh-153872: macOS: free-threading support is installed by default on Python 3.15 (GH-154103) #154479