bug#26149: SRFI-19 doc erroneously warns about Gregorian reform

[Zefram]

The documentation, near the start of the section on SRFI-19, says

!    *Caution*: The current code in this module incorrectly extends the
! Gregorian calendar leap year rule back prior to the introduction of
! those reforms in 1582 (or the appropriate year in various countries).
! The Julian calendar was used prior to 1582, and there were 10 days
! skipped for the reform, but the code doesn't implement that.
!
!    This will be fixed some time.  Until then calculations for 1583
! onwards are correct, but prior to that any day/month/year and day of the
! week calculations are wrong.

The statements that the code is incorrect in this behaviour are erroneous.
SRFI-19 itself says

# A Date object, which is distinct from all existing types, represents a
# point in time as represented by the Gregorian calendar as well as by a
# time zone.

The code is thus correct in always using the Gregorian calendar in
date structures.  Per ISO 8601 it is also correct in always using
the Gregorian calendar in string output in that standard's formats.
SRFI-19 isn't explicit about the calendar used as the basis for the
other string output formats, but since the formatting proceeds from a
date structure it seems implied that they should use the same basis as
the date structure.  For string input it is explicit that the parseable
numeric formats correspond directly to fields of the date structure.
There is no part of SRFI-19 that looks like it is ever intended to use
the Julian calendar.

So the code should not be `fixed', and the statements about that and about
incorrectness should be removed from the documentation.  It is sensible to
keep an explicit statement about the treatment of the Gregorian reform,
but the decision to use the Gregorian calendar proleptically should be
credited to SRFI-19 (the standard), not to the code.

-zefram