docs: Document that the man pages are for our builtins

For builtins that have the same name as common commands, it might not
be entirely obvious that there is another page.

So, for those builtins, we add a note, but only in the man pages.

(exception is true and false because the note would be longer than the
page, and it's fridging true and false)

Fixes #8077.

doc_src/cmds/pwd.rst

@@ -14,6 +14,12 @@ Synopsis
 Description
 -----------
 
+.. only:: builder_man
+
+          NOTE: This page documents the fish builtin ``pwd``.
+          To see the documentation on the ``pwd`` command you might have,
+          use ``command man pwd``.
+
 ``pwd`` outputs (prints) the current working directory.
 
 The following options are available: