Side inputs documentation is not clear about the model

Side input documentation (https://beam.apache.org/documentation/programming-guide/#side-inputs) does not clearly explain how the projection will happen and how it might differ between runners and sdks. (Note that the model is vague in these areas and documentation could at least explain what could be the set of expected outcomes. Unfortunately documentation could only do so much to clarify.)

/cc [~pcoet]

Imported from Jira [BEAM-12610](https://issues.apache.org/jira/browse/BEAM-12610). Original Jira may contain additional context.
Reported by: altay.