I found several fancy descriptions when skimming gmane.emacs.gnus.general newsgroup.

,----[ C-h f user-uid RET ]
| user-uid is a built-in function in ‘C source code’.
|
| (user-uid)
|
|   Probably introduced at or before Emacs version 18.
|   This function does not change global state, including the match data.
|
| Return the effective uid of Emacs.
| Value is a fixnum, if it’s small enough, otherwise a bignum.
`----

I’m impressed that `C-h v user-uid RET’ is *clickable*.

How Gnus fontifys it and how can I generate funciton/variable documentations like it?
I think it a Gnus builtin function.

Thanks.

--
Zhiwei Chen