[bug-gawk] third argument to split() always behaves like FS, and the docs are not clear about it

[R]

If the third argument to split() is made up of a single character, it
will NOT be treated as a regular expression:

$ gawk 'BEGIN{print split("foo.bar", a, "."); print a[1], a[2]}'
2
foo bar

But:

$ gawk 'BEGIN{print split("foo.bar", a, /./); print a[1], a[2]}'
8


This is (for me) completely expected, and it's the way it works in all
awk implementations I know of.

But neither the man nor the info page of gawk are clear about it --
they both talk about the 3rd argument as of a 'regular expression'.
The info page goes into some detail about the case where it's a single
space, but does not say anything about the case where it's a special
character like '.' or '('.

Here is the question and my awkward attempt to answer it that triggered this:
https://unix.stackexchange.com/a/483874/308316