DOC: management: document the "trace" and "show trace" commands

DOC: management: document the "trace" and "show trace" commands

At the moment the subsystem is still not complete and the various modules
do not yet produce traces (some dirty experimental code for H2 exists) but
this aims at easing a broad adoption.

Among the missing elements, we can enumerate the lack of configuration
of the sinks (e.g. it's still not possible to change their output format
nor enable/disable timestamps) and since timestamps are not availalbe in
the sinks, they are not collected nor passed by the traces.