mono_glue is considered unknown by SCons, while it actually exists

[cyrilbos]

Your Godot version:
master Mono
Issue description:
The mono_glue option for scons does not seem to exist anymore. I get this warning:

scons -j8 p=windows tools=yes module_mono_enabled=yes mono_glue=no
scons: Reading SConscript files ...
WARNING: Unknown SCons variables were passed and will be ignored:
mono_glue=no

I do not have the knowledge to know how to properly build Godot with C# support so I can not modify the documentation correctly.

URL to the documentation page:
https://docs.godotengine.org/en/latest/development/compiling/compiling_with_mono.html#generate-the-glue

[Calinou]

This is a regression from #55203. The option still exists, but SCons mistakenly labels it as unknown: #56359