bug#47762: Doc of `macroexp-file-name' (Emacs 28)

[Drew Adams <drew.adams@oracle.com>]

I don't have Emacs 28 code, where this function is apparently defined.
I'm going by what is shown here:

https://emacs.stackexchange.com/a/64404/105

The doc string shown there is this:

 Return the name of the file from which the code comes.
 Returns nil when we do not know.
 A non-nil result is expected to be reliable when called from a macro in order
 to find the file in which the macro's call was found, and it should be
 reliable as well when used at the top-level of a file.
 Other uses risk returning non-nil value that point to the wrong file.

Please consider clarifying the doc for this function.  For example, just
what code is meant here: "the file from which the code comes"?
                                              ^^^^^^^^

Looking at the source code doesn't help understanding of the function
either (at least in releases prior to Emacs 28).  The doc string for
variable `current-load-file' just says "Used for internal purposes by
`load'.", and there's no doc string at all for variable
`byte-compile-current-file'.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19041
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''