[1.6.x] Better migration docs for get_query_set -> get_queryset

This way they won't need to fix things again when get_query_set disappears

Backport of 91e2780dfba5609b0f83d51aee1cf7e8fe78b1f7 from master
[1.6.x] Fixed #23543 -- Added docs on testing management command output.

Backport of cdee8659763ee7044c1507bcd2202581b1744f0b from master
[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