Update Action Mailer doc [ci skip]

Update Active Storage docs [ci skip]

This is a partial revert of 07402aa1.

I also changed "cache-key" to "cache key" because "cache-key" appears
only once in the repo.

```
% git grep '\bcache-key\b' | wc -l
1

% git grep '\bcache key\b' | wc -l
28
```

I'm not sure this is the problem we're seeing on Travis (occasional
unexpected IOError exceptions), but it's worth a try.

Update Caching with Rails guide [ci skip]

Merge pull request #30451 from y-yagi/fix_cant_modify_forzen_string_error_in_dbconsole_with_sqlserver

Fix `can't modify frozen String` error in `DBConsole`

Without this, `dbconsole` raises an error as follwing:

```
RuntimeError: can't modify frozen String
    railties/lib/rails/commands/dbconsole/dbconsole_command.rb:79:in `start'
```

Added CHANGELOG for #30360 [ci skip]

Update Rails I18n API guide [ci skip]

Fix `can't modify frozen String` error in `DebugLocks`

Clarify that bulk option is supported only by MySQL

Fix indentation in the template for secrets

- Closes #30441

This fixes following warnings:

```
actionpack/lib/action_dispatch/middleware/debug_locks.rb:46: warning: shadowing outer local variable - threads
```

Without this, `DebugLocks` middleware raises an error as follwing:

```
Puma caught this error: can't modify frozen String (RuntimeError)
actionpack/lib/action_dispatch/middleware/debug_locks.rb:97:in `block in render_details'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `each'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `map'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `render_details'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:37:in `call'
railties/lib/rails/engine.rb:524:in `call'
```

If `:readers` fixture is loaded before the test, it will be failed.
Use `firm.developer_ids` instead because we don't have `:contracts`
fixture for now.

https://travis-ci.org/rails/rails/jobs/268976230#L729

Because `add_as_polymorphic_through` is only called when
`options[:source_type]` is true.

Retrive screenshot in relative path of current directory

Fix grammar in JavaScript in Rails [ci skip]

* I've fixed grammar problems in the guide about JavaScript in Rails.

`RuntimeReflection#alias_candidate` is no longer used since 0408e212.

Context: https://github.com/rails/rails/pull/29619#discussion_r125158589

I was added a table options after `force: :cascade` in #17569 for not
touching existing tests (reducing diff). But `force: :cascade` is not an
important information. So I prefer to place a table options before
`force: :cascade`.

In Rails engine `Rails.root `returns the path of the dummy application.
Therefore, there is no `tmp` directly where the test is running, so can
not get the screenshot.
For this reason, instead of directly specifying tmp, retrive screenshot by
relative path from the current directory.

Fixes #30405

Use tt in doc for ActionView [ci skip]

Use tt in doc for ActiveRecord [ci skip]

Use tt in doc for ActiveSupport and Railties [skip ci]