Cleanup Guides Guidelines [ci skip]

* Copy editing for clarity in several locations
* Rename "Titles" section to "Headings"
* Fix incorrect anchor in link to API Documentation Guidelines for filenames

[ci skip] Fix sample code in engines guide

[ci skip] Changed options to +options+ for rdoc

Pass block for logging.

Using delegated connection method

Rails Tutorial link updated

Ruby on Rails Tutorial link updated.

This follows the good practice listed on http://guides.rubyonrails.org/debugging_rails_applications.html#impact-of-logs-on-performance.

Docs: grammar fix for fetch. [ci skip]

This reverts commit 70597762, reversing
changes made to dde91e9b.

IT BROKE THE BUILD!!!

Add digest config option for SignedCookiesJar

You can now configure custom digest for cookies in the same way as `serializer`:

config.action_dispatch.cookies_digest = \SHA256'

Fixes test for Gemfile entry changes

Broken by fbe38c9e

Full stack source extract

Fixes to TestCaseTest.

Originally introduced in f9086d63, the documentation of this method is
wrong as #const_regexp returns a string to easy the interpolation inside
regular expressions.

Fix typo in plugins doc

[ci skip] Changed 'first argument' to '+name+ argument'

Provide the ability to extract the source code of the entire exception stack
trace, not just the frame raising the error. This improves debugging
capability of the error page, especially for framework-related errors.

`#create!` regarding validations in contrast to the behavior of
`#create`. Also describe creating multiple objects using an array of
hashes as the +attributes+ parameter. [ci skip] /cc #16384

[ci skip] Added +object+ and +attributes+ to create! description for rdoc

Pull spring gem entry into the Gemfile template instead of gemfile_entries so it can be grouped with the other development tools