doc: fix egregious markup output

We use '+++' syntax to output a literal '**' for a '--glob' example.
This '+++' syntax is pretty ugly when rendered literally via --help. We
fix this by hackily inserting the '+++' syntax for its one specific case
that we need it during man page generation.

Not ideal but it works. And --help still has some '*foo*' markup, but we
live with that for now.

Fixes #1581

crates/core/app.rs

@@ -1354,7 +1354,7 @@ command line takes precedence.
 When this flag is set, every file and directory is applied to it to test for
 a match. So for example, if you only want to search in a particular directory
 'foo', then *-g foo* is incorrect because 'foo/bar' does not match the glob
-'foo'. Instead, you should use *-g +++'foo/**'+++*.
+'foo'. Instead, you should use *-g 'foo/**'*.
 "
     );
     let arg = RGArg::flag("glob", "GLOB")