Make docs clear that you shouldn't change an existing onUpgrade step.

Clarifying the API docs to make it clear to the developer the once a
migration step runs, any errors in that step should be corrected by adding
a new migration step instead of modifying the existing step.

Test: Docs only change
Bug: 258831395
Change-Id: I51874bc215a7edfc270eaf4893147e452beb05d3