pgsql: 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4e23c9ef65accde7eb3e56aa28d50ae5cf79b64b

Modified Files
--------------
doc/src/sgml/filelist.sgml                 |     5 +-
doc/src/sgml/func.sgml                     | 32075 ---------------------------
doc/src/sgml/func/allfiles.sgml            |    40 +
doc/src/sgml/func/func-admin.sgml          |  2962 +++
doc/src/sgml/func/func-aggregate.sgml      |  1418 ++
doc/src/sgml/func/func-array.sgml          |   646 +
doc/src/sgml/func/func-binarystring.sgml   |   854 +
doc/src/sgml/func/func-bitstring.sgml      |   358 +
doc/src/sgml/func/func-comparison.sgml     |   638 +
doc/src/sgml/func/func-comparisons.sgml    |   336 +
doc/src/sgml/func/func-conditional.sgml    |   283 +
doc/src/sgml/func/func-datetime.sgml       |  2200 ++
doc/src/sgml/func/func-enum.sgml           |   121 +
doc/src/sgml/func/func-event-triggers.sgml |   332 +
doc/src/sgml/func/func-formatting.sgml     |  1193 +
doc/src/sgml/func/func-geometry.sgml       |  1261 ++
doc/src/sgml/func/func-info.sgml           |  3790 ++++
doc/src/sgml/func/func-json.sgml           |  3945 ++++
doc/src/sgml/func/func-logical.sgml        |   146 +
doc/src/sgml/func/func-matching.sgml       |  2471 +++
doc/src/sgml/func/func-math.sgml           |  1615 ++
doc/src/sgml/func/func-merge-support.sgml  |    78 +
doc/src/sgml/func/func-net.sgml            |   592 +
doc/src/sgml/func/func-range.sgml          |  1053 +
doc/src/sgml/func/func-sequence.sgml       |   195 +
doc/src/sgml/func/func-srf.sgml            |   306 +
doc/src/sgml/func/func-statistics.sgml     |    85 +
doc/src/sgml/func/func-string.sgml         |  1818 ++
doc/src/sgml/func/func-subquery.sgml       |   349 +
doc/src/sgml/func/func-textsearch.sgml     |  1046 +
doc/src/sgml/func/func-trigger.sgml        |   135 +
doc/src/sgml/func/func-uuid.sgml           |   188 +
doc/src/sgml/func/func-window.sgml         |   284 +
doc/src/sgml/func/func-xml.sgml            |  1283 ++
doc/src/sgml/func/func.sgml                |    84 +
35 files changed, 32109 insertions(+), 32076 deletions(-)