ENH: Documented use of \1 syntax in replace expression.

Source/cmStringCommand.h

@@ -92,7 +92,10 @@ public:
       "possible and store the matches in the output variable as a list.\n"
       "REGEX REPLACE will match the regular expression as many times as "
       "possible and substitute the replacement expression for the match "
-      "in the output.\n"
+      "in the output.  The replace expression may refer to paren-delimited "
+      "subexpressions of the match using \\1, \\2, ..., \\9.  Note that "
+      "two backslashes (\\\\1) are required in CMake code to get a "
+      "backslash through argument parsing.\n"
       "COMPARE EQUAL/NOTEQUAL/LESS/GREATER will compare the strings and "
       "store true or false in the output variable.\n"
       "ASCII will convert all numbers into corresponding ASCII characters.\n"