cmRST: match anonymous hyperlink syntax

Previously, the `cmake --help-command string` output would have an extra
trailing `_` after `string(JSON)` subcommand names because they are
anonymous hyperlinks in the source. Support matching a double underscore
so that it is removed in the `--help-*` output.

Source/cmRST.cxx

@@ -40,7 +40,7 @@ cmRST::cmRST(std::string docroot)
               "cmake-workflow-option|cpack-option|ctest-option|"
               "ctest-dashboard-option"
               "):`(<*([^`<]|[^` \t]<)*)([ \t]+<[^`]*>)?`")
-  , InlineLink("`(<*([^`<]|[^` \t]<)*)([ \t]+<[^`]*>)?`_")
+  , InlineLink("`(<*([^`<]|[^` \t]<)*)([ \t]+<[^`]*>)?`__?")
   , InlineLiteral("``([^`]*)``")
   , Substitution("(^|[^A-Za-z0-9_])"
                  "((\\|[^| \t\r\n]([^|\r\n]*[^| \t\r\n])?\\|)(__|_|))"