Grammar and formatting fixes in Form Helpers Guide

Improved version of #14851 after helpful feedback from @robin850 and @matthewd.

[skip ci]
Fix code & grammar in Form Helpers Guide

in the "Forms to External Resources" section [skip ci]
Merge pull request #14873 from jonatack/patch-8

Fix code & improve grammar in "Forms to External Resources" [ci skip]
Fix custom join_table name on habtm reflections

When used a custom join_table name on a habtm, rails was not saving it
on Reflections. This causes a problem when rails loads fixtures, because
it uses the reflections to set database with fixtures.
Merge pull request #14853 from kassio/kb-fix-custom-join-table-habtm

Fix to use custom join table in habtm
Warning removed for missmatched indentation
Fix documentation of extract_handler_and_format_and_variant [ci skip]
Merge pull request #14875 from prathamesh-sonpatki/fix-extract-handler-format-variant-doc

Fix documentation of extract_handler_and_format_and_variant [ci skip]
Merge pull request #14874 from arunagw/aa-warning-removed-missmatched

Warning removed for missmatched indentation
Merge pull request #14872 from jonatack/patch-7

Grammar and formatting fixes in Form Helpers Guide
Generators, remove leftover variable assignment. refs #14877

The last uses of this variable was removed with the `--builder` option.
  2da5ea1c9feb4a7b491bd00bd6ffd931b9203da8
Revert "Document ActionController::TestCase::Behavior::ClassMethods.process"

This reverts commit 2aef9fddf8b4487a05638ea3341dfd65ceb229ef.

Reason: The added change isn't clear in terms of explaining the
arguments. Will ask the author to provide a PR and take it forward.

[ci skip]
Revert "Running AR tests on postgres, mysql & sqlite"

This reverts commit 28bd442034e0974d0a4d664076dafb8e9555816d.

Reason: Existing text was clearer imo.

[ci skip]
Merge branch 'master' of github.com:rails/docrails
Revert "Don't rely on Arel master in bug report template [ci skip]"

This reverts commit fd6d7835.

We are back on arel master, see b395265f.
Remove double declaration of memcache services on travis
Merge pull request #14889 from kiela/master

Remove double declaration of memcache services on travis
Added a missing `end`
Merge pull request #14890 from fuzzyalej/patch-1

Added a missing `end`
write a new method to be accessed from delete_all

The delete method is very coupled with delete all even
though only a portion of the conditionals apply. Decoupling
this will make the code easier to understand and manipulate.