Mention usage error templates in mkusage() docstring

src/include-bash/pretty.sh

@@ -135,7 +135,14 @@ mkusage() {
     #
     # Optionally, you can add -w MSG to add clarifying message
     # (presented as warning) to help user understand what is
-    # wrong with arguments they have passed.
+    # wrong with arguments they have passed.  Alternatively,
+    # -m SUBJ, -M SUBJ, -u SUBJ, and -U SUBJ are shorthands for
+    # error messages describing missing positional argument (-m),
+    # missing value to a parametrized argument (-M), unknown
+    # argument (-u) or unknown command (-U).  Using these
+    # shorthands you don't need  to provide whole message, but
+    # only subject in question; rest of the message is provided
+    # by pretty.sh.
     #
     # Use "--" to delimit end of arguments processed by mkusage.
     #