[fxfs-versioning] Update guidelines for struct migration and unversioned types

This captures the conventions around unversioned type usage in
feature code, stripping out documentation from legacy variants,
and safely defining legacy item tuple sets without
migrate_nodefault conflicts.

Bug: None
Test: None, documentation change.
Change-Id: I3b75ab31fdd01d7c35ca6dbb539e0faebe6a5c1e
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1546774
Reviewed-by: Martin Lindsay <mlindsay@google.com>
Commit-Queue: Aaron Drew <ripper@google.com>