[skip ci] fix and expand file_device and file_extension documentation

[mathren]

Regarding this mailing list request: https://lists.mesastar.org/pipermail/mesa-users/2024-August/015307.html

Documentation was incorrectly suggesting pdf files can be produced directly from pgplots and the option to produce ps and eps were instead not documented.

[warrickball]

I just had a quick play around with this myself and I'm not sure it's clear that file extension is just what is literally used in the filename, has no bearing on the format and is not limited to a string set of options. So although users probably want to use an extension associated with the relevant device, it's not required.

If we only intend to write files with the relevant file extensions, we should remove the file_extension option and write code that selects the extension based on file_device. From what little I know about PGPLOT, it theoretically supports other options for file_device but I can't see us (or our users) wanting anything but ps, vcps or png. (I briefly tried to find a list of supported devices but didn't manage to find one that mentioned png, which is clearly supported.)

[pmocz]

I just had a quick play around with this myself and I'm not sure it's clear that file extension is just what is literally used in the filename, has no bearing on the format and is not limited to a string set of options. So although users probably want to use an extension associated with the relevant device, it's not required.

If we only intend to write files with the relevant file extensions, we should remove the file_extension option and write code that selects the extension based on file_device. From what little I know about PGPLOT, it theoretically supports other options for file_device but I can't see us (or our users) wanting anything but ps, vcps or png. (I briefly tried to find a list of supported devices but didn't manage to find one that mentioned png, which is clearly supported.)

Great idea! Removing the file_extension option will minimize confusion. I created a PR here: #722