string: Fix regex documentation of '^' and '$' (#14028)

They match at the beginning and end of the input, not by line.

Reported-by: Martin Perzl <[email protected]>

Source/cmStringCommand.h

@@ -129,8 +129,8 @@ public:
       "will search for the position of the last occurrence of the "
       "specified substring.\n"
       "The following characters have special meaning in regular expressions:\n"
-      "   ^         Matches at beginning of a line\n"
-      "   $         Matches at end of a line\n"
+      "   ^         Matches at beginning of input\n"
+      "   $         Matches at end of input\n"
       "   .         Matches any single character\n"
       "   [ ]       Matches any character(s) inside the brackets\n"
       "   [^ ]      Matches any character(s) not inside the brackets\n"