time: add to format documentation about dangers of using RFC1123{,Z} for parsing

When using time.RFC1123Z to parse the date header value out of an email,
an error is returned for dates that occur in the first 9 days of a
month. This is because the format strings for RFC 1123 defined in the
time package indicate that the day should be prefixed with a leading 0.

Reading the spec, the line that talks about it seems to indicate that
days can be either 1 or 2 digits:

  `date = 1*2DIGIT month 2*4DIGIT`

So a date header with a day like `7` with no leading zero should be
accepted.

Fixes #67887

Change-Id: Ie7ee40d94da2c8c0417957e8b89f9987314949c8
GitHub-Last-Rev: 22a5a52fcb
GitHub-Pull-Request: golang/go#67888
Reviewed-on: https://go-review.googlesource.com/c/go/+/591335
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/time/format.go

@@ -32,6 +32,10 @@ import (
 // only to local times. Applying them to UTC times will use "UTC" as the
 // time zone abbreviation, while strictly speaking those RFCs require the
 // use of "GMT" in that case.
+// When using the [RFC1123] or [RFC1123Z] formats for parsing, note that these
+// formats define a leading zero for the day-in-month portion, which is not
+// strictly allowed by RFC 1123. This will result in an error when parsing
+// date strings that occur in the first 9 days of a given month.
 // In general [RFC1123Z] should be used instead of [RFC1123] for servers
 // that insist on that format, and [RFC3339] should be preferred for new protocols.
 // [RFC3339], [RFC822], [RFC822Z], [RFC1123], and [RFC1123Z] are useful for formatting;