[1.6.x] Further fixes to the migration notes for get_query_set

This rename is very tricky for the case of subclasses which define
get_query_set and haven't been updated yet, which applies to all projects in
the form of RelatedManager from Django 1.5.

Backport of 0c623da66406d1f20b5e26d497d57da5ad0de066 from master