Add Notice to Signals and Slots Docs to use ::class and beware of common file mistake

[filipcoja]

Description

New developers could interpret the documentation regarding Signals and Slots (https://flowframework.readthedocs.io/en/stable/TheDefinitiveGuide/PartIII/SignalsAndSlots.html) wrong and use the file path to the class instead of the Namespace.

Resolution

We should update the example to use ClassName::classinstead of a String containting the namespace since this is considered best practice. This would also eliminate the misinterpretation of the docs.

[bwaidelich]

While we're on it we should also add a big notice about the method name that has to be prefixed with emit but referenced without the prefix

[bwaidelich]

Related: #1003