[time.format] Add commas and clarifying "unless otherwise" (#7697)

See LWG 4124, which added wording before the text in question that motivates adding "unless otherwise specified".

Author: xmcgcg

source/time.tex

@@ -10784,14 +10784,14 @@
 \\ \rowsep
 \tcode{\%y} &
 The last two decimal digits of the year.
-If the result is a single digit it is prefixed by \tcode{0}.
+If the result is a single digit, it is prefixed by \tcode{0}.
 The modified command \tcode{\%Oy} produces the locale's alternative representation.
 The modified command \tcode{\%Ey} produces the locale's alternative representation
 of offset from \tcode{\%EC} (year only).
 \\ \rowsep
 \tcode{\%Y} &
 The year as a decimal number.
-If the result is less than four digits
+If the result is less than four digits,
 it is left-padded with \tcode{0} to four digits.
 The modified command \tcode{\%EY} produces
 the locale's alternative full year representation.
@@ -10816,7 +10816,8 @@
 \end{LongTable}
 
 \pnum
-If the \fmtgrammarterm{chrono-specs} is omitted,
+Unless otherwise specified,
+if the \fmtgrammarterm{chrono-specs} is omitted,
 the chrono object is formatted
 as if by streaming it to \tcode{basic_ostring\-stream<charT> os}
 with the formatting locale imbued