Docs: lightning-ping outdated info about pings to peers with channels only

[daywalker90]

It currently only works for peers we have a channel with. is outdated.

I propose a change to A successfull connection with 'connect' is required before ping is possible.

[sangbida]

This issue is being closed as part of issue cleanup.

Reason: No recent activity (last updated 1268 days ago)

If you believe this issue should remain open, please comment and we can reopen it.