I think of these as synonymous -- do you have any particular reason to prefer "fraction"? I don't feel strongly about it either way, so I'm quite happy to go with fraction if folks find that more expressive.
+ * The caller must hold (at least) shared AysncQueueLock.
A possibly better wording: The caller must hold AysncQueueLock in (at least) shared mode.
Yes, that is more accurate.
Unnecessary whitespace changes in pg_proc.h for existing functions.
I did group the asynchronous notification functions together, which seemed reasonable as there are now three of them, and changed the tabbing between the function name and namespace ID to match, as is done elsewhere in pg_proc.h. I think those changes improve readability, but again I don't feel strongly about it.
+DESCR("get the current usage of the asynchronous notification queue");
A possibly better wording: get the fraction of the asynchronous notification queue currently in use
