Document gsub/3

docs/content/3.manual/manual.yml

@@ -2029,7 +2029,7 @@ sections:
             input: '("ab,cd", "ef, gh")'
             output: ['"ab"', '"cd"', '"ef"', '"gh"']
 
-      - title: "`sub(regex; tostring)`"
+      - title: "`sub(regex; tostring)` `sub(regex; string; flags)`"
         body: |
 
           Emit the string obtained by replacing the first match of regex in the