cal: mention where Gregorian reformation happen in September 1752

In September 1752 the Gregorian reformation happen in Great Britain and
Dominions (including what is now the USA).  One could argue the cal(1)
should consider locale when determining the reformation, but such is
nearly impossible implement correctly.

The dates when reformation happen are split by areas that cannot be
expressed with current locales, for example the Netherlands is split to
two and Germany three categories depending on where in the modern country
one is.  Secondly the track when reformation happen is lost for some
countries, Lithuania or Japan are good examples of such.

One of the worst for a programmer is Sweden.  They got gradual calendar
change which made Swedish calendar to be completely out of sync.  Later
the Swedish calendar jumped in 1753 to be in sync with everyone else.

Notice that some countries, including Saudi Arabia, Ethiopia, Nepal, Iran
and Afghanistan, still have not adopted the Gregorian calendar.  Hence
the output of the cal(1) cannot be considered to be correct for everyone.

References: http://calendopedia.com/gregory.htm
References: http://en.wikipedia.org/wiki/Gregorian_calendar#Gregorian_reform
Reviewed-by: Benno Schulenberg <bensberg@justemail.net>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
cal: in man page admit that any week has just one Sunday

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
Signed-off-by: Karel Zak <kzak@redhat.com>