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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.

--- a/sys/man/2/ctime

+++ b/sys/man/2/ctime

@@ -128,3 +128,6 @@

 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).

--- a/sys/man/2/tmdate

+++ b/sys/man/2/tmdate

@@ -275,6 +275,13 @@

 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 The timezone information that we ship is out of date.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.