[skip ci] Fix counter_cache in the Rails Guides
[skip ci] Fix counter_cache in the Rails Guides

In the AR Associations Guide, this PR fixes:

- The counter_cache declaration is now shown only in the `belongs_to`
association.

- The docs stated that the counter_cache declaration needs to be on the
`has_many` side; now corrected to the `belongs_to` side.

- Split the custom column explanation out to a separate paragraph.

- Simplify the NOTE because it is true both with and without a custom
column name.

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