Update documentation on STI change handling

Documenting a potential source of confusion about how STI classes handle changes.
Merge pull request #15987 from jamesprior/patch-2

Update documentation on STI change handling
Move STI docs off of the main Base document, leaving a note
Fix 10.3 invite_email.body assert_match example.

Currently there is no implicit conversion for Mail::Body to string
therefore .to_s must be applied. This is exampled in 10.2.2.
Merge pull request #15989 from nickpellant/master

Fix 10.3 invite_email.body assert_match documentation example.