Fix documentation regarding ModelSearchIndex to match current behavior

Closes #606. Thanks to @bradleyayers for the report.
Remove bit about SearchQuerySet.load_all_queryset deprecation

That method was entirely removed in commit b8048dc0e9e3.

Closes #607. Thanks to @bradleyayers for the report.
Merge pull request #881 from benspaulding/docs/issue-606

Fix documentation regarding ModelSearchIndex to match current behavior
Merge pull request #882 from benspaulding/docs/issue-607

Remove bit about SearchQuerySet.load_all_queryset deprecation