v2 migration algorithm docs for rewriting saved object id's (elastic#…

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* Document v2 migration algorithm using control state names from source code

* v2 migrations support for rewriting document _id's

* Minor edits and clarifications

* Fix markdown formatting

* Review comments: improve algorithm description

* add WAIT_FOR_YELLOW_SOURCE step

Co-authored-by: restrry <[email protected]>
# Conflicts:
#	rfcs/text/0013_saved_object_migrations.md