asix-sigma: discuss usability of data pattern trigger specs

When data patterns for trigger specs span multiple bits, users may not
want to specify long lists of "<ch>=<lvl>" conditions for sigrok-cli's
--trigger option, and count channels by hand. Or click a dozen dialogs
to specify one data pattern, or modify a previous specification. Setups
with few traces may accept that, "data heavy" setups like parallel data
or address bus inspection may not.

Add comments which discuss the potential use of SR_CONF_TRIGGER_PATTERN.
Outline a syntax which may be flexible enough _and_ acceptable to users,
support data patterns and edge triggers alike, in several presentations
that serve different use cases.  This commit exclusively adds comments,
does not change behaviour.

Update a comment in the user spec to internal format trigger spec parser
to expand on hardware constraints and implementation limitations. Rename
an identifier which checks the number of edge conditions, not the number
of accepted trigger spec details.