Document latex_additional_files behavior for files with .tex extension.

[mforbes]

Describe the bug

As discussed in #4090, adding files to latex_additional_files that end in .tex causes them to be appended to the latexmk command so that they get compiled automatically. This can cause problems for files that are intended to be \input{} as discussed in #4090. The resolution there was to append .tex.txt to the filename in the documentation example, however, the documentation does not actually document the behaviour for files ending in .tex.

Could the actual intended behaviour please be precisely specified in the so that users don't need to guess and check? (Are there any other special cases?)

How to Reproduce

The complete behaviour is described in #4090. Just include an incomplete file with a name ending in .tex in latex_additional_files. This files gets appended to the latexmk command line, which then attempts to compile it as if it were a complete standalone file.

This is probably the intended behaviour, but if so, please document this.

Environment Information

N/A

Sphinx extensions

No response

Additional context

No response

[jfbu]

Thanks, done at #10993. Testing fails for unrelated matters. But delaying merge because of that.