>> OTOH, there are messages like "Use \\`+',\\`-',\\`0' for further adjustment"
>> have not only a prefix, but also a suffix.
>
> Perhaps the MESSAGE should be a format-spec string?  So the caller could
> say "Use %k for further adjustment" and have the keymap description
> inserted at the %k.  That should be plenty flexible.

format-spec is a nice helper.  Here are the changes after
the value of %k is automatically generated from the keys:

  emoji-zoom-increase:
  OLD: Zoom with + and -
  NEW: Zoom with +, -

  indent-rigidly:
  OLD: Indent region with <left>, <right>, S-<left>, or S-<right>.
  NEW: Indent region with TAB, <left>, <right>, S-<left>, S-<right>

  text-scale-adjust:
  OLD: Use +,-,0 for further adjustment
  NEW: Use +, =, -, 0, C-+, C-=, C--, C-0 for further adjustment

  global-text-scale-adjust:
  OLD: Use +,-,0 for further adjustment
  NEW: Use +, =, -, 0, ESC for further adjustment

ESC is because map-keymap handles only top-level keys
but C-M-+ is [ESC C-+].