Add more documentation for create_join_table.

Explain that it doesn't create indices by default and
that it also has a block form.
Fix warning
Indent comment block by two spaces.

As requested by @fxn
https://github.com/schuetzm/rails/commit/743d15bba182df80d003d205690ead988970299c#commitcomment-2661417
remove uncommented railties boot_test.rb
Merge pull request #9352 from senny/remove_uncommented_boot_test

Remove commented railties boot_test.rb
Merge pull request #9348 from schuetzm/doc_no_indices_with_create_join_table

Add more documentation for create_join_table.