Commit 6753333f stopped using semaphores as the sleep/wake mechanism for
heavyweight locks, but some obsolete references to that scheme remained
in comments.  As with similar commit 25b93a29, back-patch all the way.
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CA%2BhUKGLafjB1uzXcy%3D%3D2L3cy7rjHkqOVn7qRYGBjk%3D%3DtMJE7Yg%40mail.gmail.com