locale.5: Document glibc conventions regarding days and week

Based on existing practice and glibc community wiki page at
https://sourceware.org/glibc/wiki/Locales

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man5/locale.5

@@ -974,14 +974,14 @@ followed by a list of abbreviated names of the days of the week.
 The list starts with the first day of the week
 as specified by
 .I week
-(Sunday by default).
+(Sunday by default). See NOTES.
 .TP
 .I day
 followed by a list of names of the days of the week.
 The list starts with the first day of the week
 as specified by
 .I week
-(Sunday by default).
+(Sunday by default). See NOTES.
 .TP
 .I abmon
 followed by a list of abbreviated month names.
@@ -1020,16 +1020,7 @@ Regarding the start of the week,
 shall be used for Sunday and
 .B 19971201
 shall be used for Monday.
-Thus, countries using
-.B 19971130
-should have local Sunday name as the first day in the
-.I day
-list,
-while countries using
-.B 19971201
-should have Monday translation as the first item in the
-.I day
-list.
+See NOTES.
 .TP
 .IR first_weekday " (since glibc 2.2)"
 followed by the number of the first day from the
@@ -1040,7 +1031,7 @@ The default value of
 corresponds to either Sunday or Monday depending
 on the value of the second
 .I week
-list item.
+list item. See NOTES.
 .TP
 .IR first_workday " (since glibc 2.2)"
 followed by the number of the first working day from the
@@ -1048,6 +1039,7 @@ followed by the number of the first working day from the
 list.
 The default value is
 .BR 2 .
+See NOTES.
 .TP
 .I cal_direction
 .\" from localedata/locales/uk_UA
@@ -1082,6 +1074,54 @@ Usual default locale archive location.
 Usual default path for locale definition files.
 .SH CONFORMING TO
 POSIX.2, ISO/IEC TR 14652.
+.SH NOTES
+The collective GNU C library community wisdom regarding
+.IR day ,
+.IR week ,
+and
+.I first_weekday
+states at
+https://sourceware.org/glibc/wiki/Locales
+the following:
+.PP
+The value of the second
+.I week
+list item specifies the base of the
+.I day
+list.
+.PP
+.I first_weekday
+specifies the offset of the first day-of-week in the
+.I day
+list.
+.PP
+For compatibility reasons, all glibc locales should set the value of the
+second
+.I week
+list item to
+.B 19971130
+(Sunday) and base the
+.I day
+list appropriately, and set
+.I first_weekday
+to
+.B 1
+or
+.BR 2 ,
+depending on whether the week actually starts on Sunday or Monday
+for the locale.
+.PP
+Thus, a locale which week starts on Monday
+.I first_weekday
+and
+.I first_workday
+should be set to
+.B 2
+and
+.I abday
+and
+.I day
+should have the locale representation of Sunday as the first list item.
 .SH BUGS
 This manual page isn't complete.
 .\" .SH AUTHOR