Teach doc/src/sgml/Makefile about the new func/*.sgml files.

Teach doc/src/sgml/Makefile about the new func/*.sgml files.

These were omitted from build dependencies and also tab/nbsp
checks, with the result that "make" did nothing after modifying
a func/*.sgml file.

Oversight in 4e23c9ef6.  AFAICT we don't need any comparable
changes in meson.build, or at least I don't see it doing anything
special for the pre-existing ref/*.sgml files.