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
Add rails:update to the upgrading guides

[ci skip]
Change double quotes to single quotes in guide
Improve consistency
Remove unneeded `@column_types` instance variable

This was used more previously, but other uses have been removed.
changelog entries for #15866 and #15848

/cc @sgrif
Delegate #model_name method to self.class
Merge pull request #15867 from sgrif/sg-rm-column-types

Remove unneeded `@column_types` instance variable
Merge pull request #15852 from SyntaxBlitz/master

Change double quotes to single quotes in guide
Add a deprecation cycle for `NullColumn` from `column_for_attribute`

This is public API, and `simple_form` depends on the `nil` return value.
We need to go through a deprecation cycle to return a null object. If
people want hash access, they can access the hash.
Small typo
Merge pull request #15880 from davetoxa/patch-1

s/overriden/overridden [ci skip]
Merge pull request #15878 from sgrif/sg-null-column

Revert #15502, and add a deprecation warning instead.
Merge pull request #15732 from kuldeepaggarwal/correct-assets-ouput

[ci skip] correct output for asset_helper methods
Merge pull request #14113 from murb/patch-1

Moved explanation of `--mountable` option up
remove unnecessary require for json serialization test cases
Merge pull request #15882 from aditya-kapoor/remove-unnecessary-require

remove unnecessary require from json serialization test cases
Remove unused param 'klass' from AttributeAssignment#read_other
Remove unused param 'conn' from ConnectionPool#release
Disable autolink in example code blocks in guides.

/cc @chancancode @robin850

configuration reference: http://alexgorbatchev.com/SyntaxHighlighter/manual/configuration/

Related issues:

* https://github.com/vmg/redcarpet/issues/334

* https://github.com/docrails-tw/guides/issues/62