docs: Document the new str.strip() feature

docs/markdown/Reference-manual.md

@@ -1430,6 +1430,8 @@ are immutable, all operations return their results as a new string.
   specified as the argument
 
 - `strip()` removes whitespace at the beginning and end of the string  
+  *(added 0.43.0)* optionally can take one positional string argument,
+  and all characters in that string will be stripped
 
 - `to_int` returns the string converted to an integer (error if string
   is not a number)