markdown-preprocess: cross-reference where opts are used

In each options/foo.md, keep a list of where the option is used.
This will be valuable to anyone making future edits, and to
those reviewing those edits.

This may be a controversial commit, because those crossref lists
are autogenerated as a side effect of the script that reads them.
It definitely violates POLA. And one day, some kind person will
reconcile (e.g.) --label, using it in more man pages, and maybe
forget to git-commit the rewritten file, and CI will fail.

I think this is a tough tradeoff, but worth doing. Without this,
it's much too easy for someone to change an option file in a way
that renders it inapplicable/misleading for some podman commands.

Signed-off-by: Ed Santiago <santiago@redhat.com>

docs/source/markdown/options/user.md

@@ -1,3 +1,7 @@
+####> This option file is used in:
+####>   podman create, exec, run
+####> If you edit this file, make sure your changes
+####> are applicable to all of those.
 #### **--user**, **-u**=*user[:group]*
 
 Sets the username or UID used and, optionally, the groupname or GID for the specified command. Both *user* and *group* may be symbolic or numeric.