doc added for `writer` method in `alias_method_chain`[ci skip]
[ci skip] Clarify deletion strategies for collection proxies

For detailed testing of behavior see:
https://gist.github.com/eileencodes/5b0a2fe011dcff6203fe

This shows destroy_all always destroys records and fires callbacks.
It will never use nullify or delete_all

delete_all's behavior varies greatly based on `hm` vs `hm:t` and deletion
strategy.
Merge pull request #17176 from kuldeepaggarwal/doc-alias_method_chain

doc added for `writer` method in `alias_method_chain` [ci skip]
Merge pull request #17179 from eileencodes/clarify-delete-strategies-on-collection-proxy

[ci skip] Clarify deletion strategies for collection proxies
Add Active Model to guides [ci skip]
Edits to ActiveModel guide [ci skip]

- Some grammatical errors and rewording of sentences throughout document

- Renaming headers to be more human readable, and if following paragraphs do not include module
  names, adding those in

- In ActiveModel::AttributeMethods
  * formatting expected output correctly, changing from `#` to `# =>`

- In ActiveModel::Dirty
  * changing expected output of person.first_name_was from "First Name" to nil
  * removing 'active_model' requirement at top of code snippet

- In ActiveModel::Validations
  * originally Person.new(token: "2b1f325") raises an argument error (1 for 0), so setting of
    attribute occurs on the line after object instantiation

- In ActiveModel::Naming
  * closing code snippet tag at end of section so ActiveModel::Model is a separate section

- In ActiveModel::Model
  * fixing spelling mistake of attr_accessor

- In ActiveModel::Serialization
  * adding edit to description of how from_json and from_xml works under serialization

- In ActiveModel::Lint::Tests
  * capitalizing person for class name