Add docs for AS::Notifications::Event#duration

[ci skip]
Fix docs for AS::Notifications::Event#duration

Stems from comments by @zzak on e07f3dd:

* https://github.com/rails/docrails/commit/e07f3ddcac394d2a8dc23fc571318b7e8c2497b1#commitcomment-9015634
* https://github.com/rails/docrails/commit/e07f3ddcac394d2a8dc23fc571318b7e8c2497b1#commitcomment-9015639

[ci skip]
Add docs for Time#find_zone

Also improves docs for `Time#find_zone!`

[ci skip]
Add docs for Numeric#*_bytes methods

Add docs for `kilobytes`, `megabytes`, `gigabytes`, `terabytes`,
`petabytes` and `exabytes`. Fix docs for `bytes`.

[ci skip]
Replace `#=>` with `# =>` [ci skip]

@rafaelfranca suggested in f7c7bcd9 that code examples should display
the result after  `# =>` and not after `#=>`.

This commit replaces *all* the occurrences of `#=>` in the code documentation
(mostly added by me :sob:) with the suggested `# =>`.
Add docs for Numeric time-related methods

Add docs for `minutes`, `hours`, `days`, `weeks` and `fortnights`.
Fix docs for `in_milliseconds`.

[ci skip]
Add docs for ActionDispatch::Http::URL methods

Add docs for `extract_domain`, `extract_subdomains`, `extract_subdomain`.

Add doc examples for `url`, `protocol`, `raw_host_with_port`, `host`,
`host_with_port`, `port`, `standard_port`, `standard_port?`, `optional_port`,
`port_string`.

[ci skip]
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.
Replace guides.ror.org/v4.2.0 with guides.ror.org

No need to point links to http://guides.rubyonrails.org/v4.2.0/... now
that the Ruby on Rails guides point to 4.2.0.
Merge pull request #18110 from claudiob/point-guide-to-master

Replace guides.ror.org/v4.2.0 with guides.ror.org
Merge branch 'master' of github.com:rails/docrails
pass over the autoloading guide [ci skip]
Renaming the autoloading guide [ci skip]

The term 'constant autoloading' can be mistakenly
construed as 'constantly autoloading'.
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.