bug#62673: 28.2; The example in "(elisp) Prefix Keys" is wrong

[Lin Jian]

Reproducing steps:
1. emacs -Q
2. eval the example in the *scratch* buffer
  2.1. (use-local-map (make-sparse-keymap))
           => nil ;; good
  2.2. (local-set-key "\C-p" ctl-x-map)
           => (keymap ...) ;; bad: manual says it returns nil
  2.3. (key-binding "\C-p\C-f")
           => find-file ;; good
  2.4. (key-binding "\C-p6")
           => 2C-command ;; bad: manual says it returns nil