Clarify documentation of ceil/ceiling/floor functions.

Document these as "nearest integer >= argument" and "nearest integer <=
argument", which will hopefully be less confusing than the old formulation.
New wording is from Matlab via Dean Rasheed.

I changed the pg_description entries as well as the SGML docs.  In the
back branches, this will only affect installations initdb'd in the future,
but it should be harmless otherwise.

Discussion: <CAEZATCW3yzJo-NMSiQs5jXNFbTsCEftZS-Og8=FvFdiU+kYuSA@mail.gmail.com>