doc: de-“note” the documentation

doc: de-“note” the documentation

* doc/coreutils.texi, man/readlink.x, man/runcon.x:
* src/comm.c (usage):
* src/digest.c (usage):
* src/echo.c (usage):
* src/join.c (usage):
* src/ln.c (usage):
* src/rm.c (usage):
* src/stat.c (usage):
* src/system.h (USAGE_BUILTIN_WARNING):
* src/test.c (usage):
* src/touch.c (usage):
* src/uniq.c (usage):
Rewrite to avoid most uses of “Note that” and similar wording.
These circumlocutions are rarely needed, and avoiding them
improves readability and lessens preaching.