Moved explanation of `--mountable` option up

The example code uses `--mountable`. In previous version this was followed by an explanation of the `--full` option, which doesn't make sense. Later on in the text this is better explained. I moved this explanation up without repeating too much.
[ci skip] correct output for asset_helper methods
    see cc255d3
Added missing colon in asset manifest JSON.
Add to guides app and helper objects on command line

[ci skip]
Add rails:update to the upgrading guides

[ci skip]
Merge pull request #15809 from maurogeorge/guides-console-app

Add to guides app and helper objects on command line
docs, configurations method is listed in the docs. [ci skip]

The :singleton-method: directive is printed in the docs.
This directive is not necessary as the method is documentable just fine.
Make dependencies.rb add a name to NameError
Add regression test for NameError#name
Merge pull request #15764 from arthurnn/dependencies_name_error

Make dependencies.rb add a name to NameError
Return a null object from `AttributeSet#[]`
[ci skip] Add Tip for change_column_null and change_column_default in guides
Merge pull request #15624 from aditya-kapoor/add-warnings-on-guides

[ci skip] Add Tip for change_column_null and change_column_default in guides
:nail_care: wrap the tip from #15624 at 80 chars [ci skip]
Move `attributes` to the `AttributeSet` object.
Add missing test cases for `attribute_method?`
Change double quotes to single quotes in guide
Improve consistency
Use with_parse_json_times helper in tests.
Avoid hardcoded magic number in test teardown.
Merge pull request #15857 from zuhao/refactor_activesupport_encoding_test

Avoid hardcoded magic number in test teardown.