Add more documentation for create_join_table.

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

As requested by @fxn
https://github.com/schuetzm/rails/commit/743d15bba182df80d003d205690ead988970299c#commitcomment-2661417
Merge pull request #9348 from schuetzm/doc_no_indices_with_create_join_table

Add more documentation for create_join_table.