Improve documentation around historical calendar rules.

Improve documentation around historical calendar rules.

Get rid of section 8.5.6 (Date/Time Internals), which appears to confuse
people more than it helps, and anyway discussion of Postgres' internal
datetime calculation methods seems pretty out of place here.  Instead,
make datatype.sgml just say that we follow the Gregorian calendar (a bit
of specification not previously present anywhere in that chapter :-()
and link to the History of Units appendix for more info.  Do some mild
editorialization on that appendix, too, to make it clearer that we are
following proleptic Gregorian calendar rules rather than anything more
historically accurate.

Per a question from Florence Cousin and subsequent discussion in
pgsql-docs.