Doc: improve description of allowed spellings for Boolean input.

datatype.sgml failed to explain that boolin() accepts any unique
prefix of the basic input strings.  Indeed it was actively misleading
because it called out a few minimal prefixes without mentioning that
there were more valid inputs.

I also felt that it wasn't doing anybody any favors by conflating
SQL key words, valid Boolean input, and string literals containing
valid Boolean input.  Rewrite in hopes of reducing the confusion.

Per bug #15836 from Yuming Wang, as diagnosed by David Johnston.
Back-patch to supported branches.

Discussion: https://postgr.es/m/15836-656fab055735f511@postgresql.org