Explain how PromptBuilder, OutputAdapter, ConditionalRouter differ in waiting for other inputs to be received before running

[julian-risch]

Is your feature request related to a problem? Please describe.
For users it can be unclear if PromptBuilder waits for inputs before running or if it runs once the first input is received. Similarly, the behavior of OutputAdapter and ConditionalRouter can be difficult to understand.

Describe the solution you'd like
We could extend the docs page of PromptBuilder and explain the parameters required_variables and variables of the component's init method and how it's decided if the component is ready to run. The latter can be done for the other two components too. https://docs.haystack.deepset.ai/docs/promptbuilder
We could also extend the docs page about pipelines and explain how it's decided when a component runs in general. https://docs.haystack.deepset.ai/docs/pipelines#data-flow
We could extend the docstrings of the components and explain how it's decided if the component is ready to run.

Describe alternatives you've considered

Additional context
@mathislucka brought up this topic

Technically, the difference of the three components is in how the set input types,
component.set_input_type(self, var, Any) / component.set_input_types(self, **{var: Any for var in input_types})