FAQ: known limitations

[Bromeon]

It probably makes sense to explicitly mention library limitations out of our control, as mentioned by @chitoyuu in godot-rust/gdnative#905 (comment).

Examples:

• Add instructions to use Cargo to set GODOT4_BIN to relative path. book#20
• 2nd-class-citizen native types: Crash when assigning a Rust instance to a variable with a type gdnative#905 (comment)

Let's use this issue to collect more candidates for such a page.