The answer, as far as I know, to your questions are "You cannot know" and "There is no such list of libraries."
apropos and similar commands are based only on what is currently loaded or autoloaded, not what is distributed with Emacs. Unless something is (a) built in (i.e., in C code), (b) available from an already loaded library, or (c) autoloaded, it is not picked by such commands.
In that case, your only resort is to search the source code (as far as I know). Grepping is one way to do that.
If a variable such as
auto-revert-use-notify (which is a user option) is explicitly mentioned in the Emacs NEWS (
C-h N), then it would be great if that mention were linked to the source library that defines it, so you could just hit
RET or click
mouse-2 on its name to open the library file to its definition. You might want to use
M-x report-emacs-bug to file that as an enhancement request.
(It might also be useful to have a command to do that in general: look up a variable's or a functions's definition in the distributed source code, without the library needing to be loaded. In this case, you knew the exact variable name, but had to
grep to find where it is defined.)