Split func.sgml into more manageable pieces

Split func.sgml into more manageable pieces

func.sgml has grown over the years to the point where it is very
difficult to manage. This commit splits out each sect1 piece into its
own file, which is then included in the main file, so that the built
documentation should be identical to the pre-split documentation. All
these new files are placed in a new "func" subdirectory, and the
previous func.sgml is removed.

Done using scripts developed by:

Author: jian he <jian.universality@gmail.com>

Discussion: https://postgr.es/m/CACJufxFgAh1--EMwOjMuANe=VTmjkNaZjH+AzSe04-8ZCGiESA@mail.gmail.com