Document Active Storage attachment and attachments field generators [ci skip]
Document Active Storage attachment and attachments field generators [ci skip]

Found while debugging issue: #42380

I was under an assumption that we have to manually add
`has_one_attached :<name>` and `has_many_attached :<name>`
in the model.

In issue #42296, user was trying to do something similar.

This PR documents the attachment and attachments field
generator to make it more visible to all people.

kddeisz/rails,Stellenticket/rails,yawboakye/rails,esparta/rails,BlakeWilliams/rails,lcreid/rails,kddeisz/rails,georgeclaghorn/rails,Vasfed/rails,Vasfed/rails,kmcphillips/rails,esparta/rails,kmcphillips/rails,Envek/rails,rails/rails,joonyou/rails,BlakeWilliams/rails,georgeclaghorn/rails,georgeclaghorn/rails,yawboakye/rails,Erol/rails,kddeisz/rails,kmcphillips/rails,EmmaB/rails-1,BlakeWilliams/rails,yahonda/rails,yawboakye/rails,EmmaB/rails-1,Stellenticket/rails,EmmaB/rails-1,Envek/rails,Edouard-chin/rails,Erol/rails,lcreid/rails,rails/rails,Edouard-chin/rails,Envek/rails,yahonda/rails,joonyou/rails,joonyou/rails,joonyou/rails,Erol/rails,kddeisz/rails,Vasfed/rails,Erol/rails,MSP-Greg/rails,yahonda/rails,Stellenticket/rails,esparta/rails,georgeclaghorn/rails,lcreid/rails,yawboakye/rails,Edouard-chin/rails,MSP-Greg/rails,esparta/rails,Vasfed/rails,rails/rails,yahonda/rails,MSP-Greg/rails,lcreid/rails,MSP-Greg/rails,kmcphillips/rails,BlakeWilliams/rails,rails/rails,Edouard-chin/rails,Envek/rails,Stellenticket/rails