Document that symmetric migrations in a YAML are parsed into pairs of `AsymmetricMigration`s

[grahamgower]

This info doesn't seem to appear anywhere. Not sure where it should go.

Mabye we should rename AsymmetricMigration to Migration (keeping AsymmetricMigration) as an alias? Then it might be natural to mention in the Migration docs that there is no separate class for symmetric migrations, as all migrations are resolved into asymmetric migrations.

[molpopgen]

Maybe an "implementation details" page could be a grab bag for this kind of stuff.