man: Clarify day-of-month format in deb-changelog(5)

Specify that the day-of-month accepts an optional leading zero, and
the amount of supported spaces.

Ref: #971977
Reported-by: Axel Beckert <abe@debian.org>

man/deb-changelog.pod

@@ -127,7 +127,8 @@ B<Mon>, B<Tue>, B<Wed>, B<Thu>, B<Fri>, B<Sat>, B<Sun>.
 
 =item I<dd>
 
-Is a one- or two-digit day of the month (B<01>-B<31>).
+Is a one- or two-digit day of the month (B<01>-B<31>), where the leading
+zero is optional, but conventionally does not get omitted.
 
 =item I<month>
 
@@ -168,6 +169,9 @@ The “trailer” line with the maintainer and date details must be
 preceded by exactly one space (U+0020 B<SPACE>).
 The maintainer details and the date must be separated by exactly two
 spaces (U+0020 B<SPACE>).
+Each part of the I<date> can be separated by one or more spaces
+(U+0020 B<SPACE>), except after the comma where it can be separated
+by zero or more spaces (U+0020 B<SPACE>).
 
 Any line that consists entirely (i.e., no leading whitespace) of B<#>
 or B</* */> style comments or RCS keywords.