Always use parentheses when referring to functions in manual

[tetrapod00]

Your Godot version:
4.4

Issue description:

In the engine, the class reference includes () in links to methods. In the online class reference, () is not included. In the manual, it's currently mixed whether we include () or not in links and mentions of functions.

Including () is definitely correct to increase clarity, and it's overall currently favored.

We should write down the standard that we include parentheses when referring to functions in the manual. Changing individual instances can't be done all at once, however.

URL to the documentation page (if already existing):
Writing Guidelines > Classes, properties, and methods.