Make sure that immutability in unit testing (Scenario) is documented

[dwilding]

Per https://github.com/canonical/operator/issues/1671, we should make sure the docs are clear about:

• Objects should be treated as immutable
• How to create adjusted objects during a unit test (see Tony's comment for examples)
• Why you might want to run more than one event as part of the same unit test, along with mutability considerations (referring back to the above two points)

[benhoyt]

@dwilding Want to take this on as Friday item? Seems small but probably still worth doing.

[dwilding]

Sounds good. My usual process for Friday is to chip away at the ones labelled docs, so that works 🙂