doc: add a statement in the documentation.

PR-URL: #35585
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

doc/api/string_decoder.md

@@ -71,6 +71,7 @@ substitution characters appropriate for the character encoding.
 
 If the `buffer` argument is provided, one final call to `stringDecoder.write()`
 is performed before returning the remaining input.
+After `end()` is called, the `stringDecoder` object can be reused for new input.
 
 ### `stringDecoder.write(buffer)`
 <!-- YAML