Ensure that .process return the method return
several enhancements to humanize [closes #12288]

* Strips leading underscores.
* Changes some unnecessary gsub!s to sub!s.
* Replaces some anchors ^, $ with \A, \z.
* Documents that human inflection rules are applied.
* Documents that words are downcased except acronyms.
* Adds an example with an acronym.
* Rewords docs.
Include label value in i18n attribute lookup

Previously, only the object and method name from the label tag were
used when looking up the translation for a label. If a value is
given for the label, this ought to be additionally used. The
following:

    # form.html.erb
    <%= form_for @post do |f| %>
      <%= f.label :type, value: "long" %>
    <% end %>

    # en.yml
    en:
      activerecord:
        attributes:
          post/long: "Long-form Post"

Used to simply return "long", but now it will return "Long-form
Post".
Merge pull request #14998 from arthurnn/tests_for_process_return

Ensure that .process return the method return
Merge pull request #14997 from jpcody/nested_value_i18n

Include label value in i18n attribute lookup
typo [ci skip]
fix guides mobile nav issue [ci skip]
Merge pull request #15000 from gregmolnar/guides_fix

fix guides mobile nav issue
add a test for the parameters sent to the url helper for new records
make sure url_for works similar to polymorphic_url
apparently passing a string to polymorphic_url is something we support

o_O
[ci skip] Add feature section and normalize prologue with other guides.

Also change Json => JSON.

Fill Ruby's Range & IPAddr link.
Merge pull request #15004 from JuanitoFatas/patch/ar-pg

[ci skip] Add feature section and normalize prologue with other guides.
Update links to new rails version in guides [ci skip]
rename `str` to `string` to fix overshadowing of variable
Follow-up to #14990 [ci skip]

Rewrite to avoid 'we'/'you', add missing period, and keep lines at 80 chars. Cheers :)

Improve readability with help from @senny

[ci skip]
Merge pull request #14993 from jonatack/patch-7

Follow-up to #14990 [ci skip]
Merge pull request #15007 from vipulnsward/fix-scrubber-overshadow

rename var to fix overshadowing of variable
Modify assert conditions not to be affected
by each database bind variable naming conventions