Done the following changes:
 1. Replace uniq call in example with autosave as uniq is not supported
 on has_and_belongs_to_many
 2. Add :readonly option to list of supported call as per
 [EdgeApi](http://edgeapi.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many)

commit a1bb6c8b which changes the naming). I've also provided an
example of how distinctness when inserting into an association can be
achieved.

Based on information here: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

It's obvious that this is a guide, and none of the other guides say 'A Guide to'.

Talked with @fxn about this. Bullet points should have periods at the ends.

This reverts commit 09682e9e.

Conflicts:
	guides/source/en/association_basics.md

REASON: Translation work can not be done in the docrails repository

This reverts commit f5b9ed4f.

REASON: Translation work can not be done in the docrails repository

This note came about due to discussion in #6786.

I've personally seen too many newbies get this wrong and pluralize the association

This reverts commit 8cb94604.

Reason: #7377 was merged and it fixes this behavior.

Fixed bug in the example code. A bit related to rails issue in
 https://github.com/rails/rails/issues/3223