Document peers/channels connection resource-control interactions with 0conf/JIT channels

[ariard]

Future deployment of advanced uses (e.g option_zeroconf and JIT channels) might have to be more conservative with their peers connection/channels opening. Can be documented better, see #1988 (comment) and #1988 (comment)

[TheBlueMatt]

If its just a documentation question, let's get it right in #1988, rather than leaving it as a followup.

[ariard]

Addressed with #1988 (comment).