Document the process of parsing command-line arguments

[Xrayez]

Parsing is not so trivial with current limitations, so worth documenting now than never, even if #26213 gets implemented.

The instructions are taken from a Q&A answer provided by @Calinou, so added as co-author. 🙂

[Calinou]

Since Godot 3.2, it should now be possible to parse command-line arguments without requiring a = between the key and value thanks to #33149. The parsing code should be modified to allow both --key value and --key=value forms (if that's not too complex) or just --key value.

[Xrayez]

I guess it's time to replace/rewrite my existing parser then. 👀

I'll come back to this once I overhaul the parsing logic (or feel free to document this yourself).

This is what Gut does for command-line parsing.

[Calinou]

@Xrayez To be fair, we could merge your PR as-is. A complete robust command-line parser can't be implemented in a few lines of code. It might be worth adding a note that this parser is a minimal implementation and may not suit all use cases.

[Xrayez]

Yeah I updated the PR to remove the obsolete information at least.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.3.