doc: clarify the operation of the comm -123 parameters

doc: clarify the operation of the comm -123 parameters

* src/comm.c (usage): give more information on the -123 parameters,
with examples to show that they can be combined.
Addresses <http://savannah.gnu.org/bugs/?24974>.
* doc/coreutils.texi (comm invocation): Mention that the
column separators are suppressed along with the column.
Suggestion from Dan Jacobson.
* man/comm.x: Reference other commands that match adjacent lines.
* man/join.x: ditto.
* man/uniq.x: ditto.