ctime(2), tmdate(2): document ctime/tmparse incompatibility

Tmparse and ctime don't mix. The timezone name may be
too long for the number of characters ctime mandates.
Document this footgun.

sys/man/2/ctime

@@ -128,3 +128,6 @@ Daylight Savings Time is ``normal'' in the Southern hemisphere.
 These routines are not equipped to handle non-\c
 .SM ASCII
 text, and are provincial anyway.
+.br
+These routines may garble the date when passed a date parsed with
+.IR tmparse (2).

sys/man/2/tmdate

@@ -275,6 +275,13 @@ Checking the timezone name against the local timezone is a
 dirty hack. The same date string may parse differently for
 people in different timezones.
 .PP
+Tmparse and ctime don't mix.
+Tmparse preserves timezone names, including names like '+0200'.
+Ctime expects timezone names to be exactly three characters.
+Use the
+.I %τ
+format character instead of ctime.
+.PP
 The timezone information that we ship is out of date.
 .PP
 The Plan 9 timezone format has no way to express leap seconds.