Always use parentheses when referring to functions in manual

**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](https://docs.godotengine.org/en/latest/contributing/documentation/docs_writing_guidelines.html#classes-properties-and-methods).