Allow emphasising placeholders in option directives (#10366)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

doc/usage/restructuredtext/domains.rst

@@ -1750,6 +1750,9 @@ There is a set of directives allowing documenting command-line programs:
    referenceable by :rst:role:`option` (in the example case, you'd use something
    like ``:option:`dest_dir```, ``:option:`-m```, or ``:option:`--module```).
 
+   Use :confval:`option_emphasise_placeholders` for parsing of
+   "variable part" of a literal text (similarly to the ``samp`` role).
+
    ``cmdoption`` directive is a deprecated alias for the ``option`` directive.
 
 .. rst:directive:: .. envvar:: name