Tweak @GDScript documentation overall

[Mickeon]

• Made use of [param] more frequently,
• Link to other classes' documentation more often.
• Improve and add more examples.
• Made the writing style closer to how the rest of the documentation is formatted.
• Ensure all of these are called "functions", not "methods". (methods are associated with objects, functions are not)

Most notably, removed "It must be a static string, so format strings can't be used." from assert(), as this behavior is actually a bug.

[kleonc]

Regarding the code examples these should be also unified to either use print or not.
Either:

print(something) # Prints ...

or

something # Returns ...

[Mickeon]

Updated the PR with most of the proposed fixes.

Regarding the code examples these should be also unified to either use print or not.

It seems like this document is not the only one plagued by this minor inconsistency. Not that it is that big of a deal, and both can be understood.

I feel like for most functions I would favour saying "Return" because it makes the code more readable. With "print(), a nested function doesn't look nearly as readable, but it can be better in some cases, such as when showing what a variable ends up equaling to (print(a))

But it could be ideal to clean it up another time. I don't think I've seen an agreed reason to use one or the other.

[mhilbrunner]

As with the other one, thanks for contributing to these docs, and thanks to kleonc for the thorough reviews!