Clarity start parameter

I find that `Specifies the starting point for the batch processing.`
does not give enough information for me to understand what this
parameter actually does.
Merge pull request #18092 from nippysaurus/clarity_batch_starting_point

Update description for `start` parameter.
Fixed syntax error in RDoc directive
Update comment regarding test environment with help from @matthewd [ci skip]
[ci skip] Improve grammar in configuring serve_static_files guide
Describe the mailer and helpers directories which are generated by `rails new`.

[ci skip]
Merge pull request #18113 from eileencodes/fix-grammar-in-assets-configuring-guide

Improve grammar in configuring serve_static_files guide [ci skip]
Fix syntax error with RDoc directive,
this should be `:nodoc:` in order to be parsed.
Remove use of "very" [ci skip]
Remove YAML implementation detail, as we're not trying to document all of
YAML's behavior in this guide. [ci skip]
Add note about the association between the two fixture files and how they're connection.

[ci skip]
Make sure this section is rendered as a `NOTE` and remove redundant sentence.

[ci skip]
"In PostgreSQL, just superusers.." => "In PostgreSQL, only superusers.."

[ci skip]
s/folder/directory [ci skip]
Shorten the explaination of this to fit on one line [ci skip]
Fixture data is accessible by a local method, and not a variable [ci skip]
Follow-up to d9710212 [skip ci]
Merge pull request #18114 from jonatack/patch-8

Follow-up to d971021 [skip ci]
We will be using the application already created in the Getting Started guide.

[ci skip]
Add 'require' for Hash#with_indifferent_access

ActiveJob::Arguments uses Hash#with_indifferent_access.
But, activejob gem does not require Hash extension library.
When we use activejob as standalone,
we need extra require statement. This is unhandy.

This commit fixes it.