Add explaination for choice of branch on release_count_.
Add explaination for choice of branch on release_count_.

Most huge pages will not be subreleased, especially if the entire application
is not using subrelease altogether.

PiperOrigin-RevId: 359841496
Change-Id: Ice909914081acdbd2760b3615ae26476ed494aa6

google/tcmalloc,google/tcmalloc,google/tcmalloc