MINOR: debug/cli: replace "debug dev counters" with "debug counters"

MINOR: debug/cli: replace "debug dev counters" with "debug counters"

"debug dev" commands are not meant to be used by end-users, and are
purposely not documented. Yet due to their usefulness in troubleshooting
sessions, users are increasingly invited by developers to use some of
them.

"debug dev counters" is one of them. Better move it to "debug counters"
and document it so that users can check them even if the output can look
cryptic at times. This, combined with DEBUG_GLITCHES, can be convenient
to observe suspcious activity. The doc however precises that the format
may change between versions and that new entries/types might appear
within a stable branch.