man page checker: enforce stricter options format

Followup to #14906, in which a nonexistent option was found
in a man page. The xref script was designed to catch that,
but I was too lax in my parsing: the option was documented
using wrong syntax, and the script didn't catch it.

Solution: do not allow *any* unrecognized cruft in the
option description lines. And fix all improperly-written
entries to conform to the rule:

    **--option**=*value(s)*

Two asterisks around option, which must have two dashes. One
asterisk around value(s).

This is going to cause headaches for some people adding new
options, but I don't think I can fix that: there are many
factors that make an unparseable line. Adding 'hint' code
would make the script even more complex than it is. I have
to assume that our contributors are smart enough to look
at surrounding context and figure out the right way to
specify options.

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

docs/source/markdown/podman-machine-init.1.md

@@ -62,7 +62,7 @@ Memory (in MB).
 
 Start the virtual machine immediately after it has been initialized.
 
-#### **--rootful**=*true|false*
+#### **--rootful**
 
 Whether this machine should prefer rootful (`true`) or rootless (`false`)
 container execution. This option will also determine the remote connection default