Clarify automatic creation/deletion of join models for has_many through

[cllns]

Fix a couple formatting things (missing : before code block, and capitalization after code block), and clarify the language.

[rails-bot]

r? @eileencodes

(@rails-bot has picked a reviewer for you, use r? to override)

[eileencodes]

Hey @cllns thanks for the PR! I think the anchor is incorrect, I think it should be going to #the-has-many-association. Can you check on that?

[cllns]

I think it's correct! Since the code specifically refers to a helper method, I think it makes more sense to link down to the list of helper methods. I could switch it to #methods-added-by-has-many, but they're basically the same (since it's only a few lines below what #has-many-association-reference) and pointing to #has-many-association-reference provides some useful context.

For reference:

• #has-many-association-reference
• #the-has-many-association
• #methods-added-by-has-many

[eileencodes]

Thanks I couldn't find it, wasn't sure where it was pointing too.