add notes and warnings to ProvideInherent docs (#9730)

* add notes and warnings to ProvideInherent docs

* rephrase ProvideInherent doc comments

* more comment refinement

* remove multiple inherents note

* remove repetition

Co-authored-by: Bastian Köcher <[email protected]>

* replace inherent example in docs

* add note about who checks is_inherent_required

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

Co-authored-by: Bastian Köcher <[email protected]>