Document the ping() functionality in manual

[evilmav]

The manual chapter Developing a Network Device Support Package should probably hint the user to implement a ping() method to avoid the controllers being restarted by the ctlmgr.

Futhermore, a log entry like WARNING:ctlmgr(artiq-host):artiq_comtools.ctlmgr:Controller xyz ping failed is misleading to an unsuspecting user as it leaves the impression that the controller is hung, where it in fact very much alive but failed to implement the ping() method. It would be nice to make the message explicit if the controller did not find the method in the first place.

[charlesbaynham]

Finding that feature is a bit of a rite of passage for new artiq users. But you're right, that should probably be fixed 😌

[dnadlinger]

We could just add an explanatory paranthetical to the error message, something like (controller dead, or ping(), which should return True, not implemented).

Mentioning this in the manual would of course also help.

@evilmav: Do you want to open a PR improving the wording?