cal: improve the help text and man page

cal: improve the help text and man page

* misc-utils/cal.c: Add a little doc string, and mention that the
  current month is implicit if not specified.  Also remove mention
  of "current" from two option descriptions as a specific date may
  have been specified.
* misc-utils/cal.1: Likewise.

[kzak@redhat.com: - use fputs and USAGE_SEPARATOR]

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