How to add syntax-highlighting to a Help buffer?

[Raffaele Ricciardi]

I would like to add some syntax-highlighting to Help buffers. I have
tried to do that for example with the Help buffer displayed by C-h f
read-string RET, by following the usual way:

(font-lock-add-keywords
 nil
 `(("\\(arbitrary regexp\\)" 1 'font-lock-function-name-face)))

But the above expression breaks the existing highlighting (the
highlighting of the function arguments disappears).  Also, now C-- C-x
C-q M-: (insert (propertize "test" 'face 'font-lock-warning-face)) RET
does not insert colorized text as it would do with a fresh Help buffer.

I am using GNU Emacs 24.5.

Thank you.