string: Clarify regex documentation of '-' behavior

Mention that it is only special inside brackets.  Show an example.

Suggested-by: Alan W. Irwin

Source/cmStringCommand.h

@@ -133,7 +133,8 @@ public:
       "   .         Matches any single character\n"
       "   [ ]       Matches any character(s) inside the brackets\n"
       "   [^ ]      Matches any character(s) not inside the brackets\n"
-      "    -        Matches any character in range on either side of a dash\n"
+      "    -        Inside brackets, specifies an inclusive range between\n"
+      "             characters on either side e.g. [a-f] is [abcdef]\n"
       "   *         Matches preceding pattern zero or more times\n"
       "   +         Matches preceding pattern one or more times\n"
       "   ?         Matches preceding pattern zero or once only\n"