doc: util.debuglog callback

[bmeck]

Checklist

• documentation is changed or added
• commit message follows commit guidelines

[bmeck]

API came from #32260

[mscdex]

I don't think it's really worth documenting this as it's purely an (internal) optimization. The callback isn't for determining whether debugging is enabled, in fact the argument passed to the callback is always a function. The callback is used to get a reference to a more optimized (by V8) debug function.

[bmeck]

@mscdex it is exposed publicly so it seems like it should be documented. will fixup the docs though

[Trott]

@bmeck There are still a two or three things flagged in the GitHub interface that look like they are worth addressing. (Can't assign the new value to debuglog because it's a const, for example.)

[bmeck]

@targos those should be fixed now, thanks

[jasnell]

Landed in 16f3b6a

[mhdawson]

LGTM