readline,doc: remove unnecessary sentence from readline doc

Author: Trott

doc/api/readline.markdown

@@ -42,8 +42,7 @@ the following values:
  - `historySize` - maximum number of history lines retained. Defaults to `30`.
 
 If a `completer` function is supplied, the user will not be able to enter tab
-characters as part of their input. If no `completer` function is supplied, tabs
-will be permitted in user input.
+characters as part of their input.
 
 The `completer` function is given the current line entered by the user, and
 is supposed to return an Array with 2 entries: