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.
Change double quotes to single quotes in guide
Improve consistency
Merge pull request #15860 from aidanharan/docrails-assets-manifest-typo

[ci skip] Added missing colon in asset manifest JSON
Remove old deprecation warning

This has been around for a couple of versions now, a `NoMethodError`
should suffice at this point.
`reload` should fully reload attributes

`reload` is meant to put a record in the same state it would be if you
were to do `Post.find(post.id)`. This means we should fully replace the
attributes hash.
Add missing test cases for `attribute_names` instance method

There is a class method with the same name that is tested, but not the
instance method.
Merge pull request #15866 from sgrif/sg-reload-attributes

`reload` should fully reload attributes
Merge pull request #15865 from sgrif/sg-missing-test

Add missing test cases for `attribute_names` instance method
Remove unneeded `@column_types` instance variable

This was used more previously, but other uses have been removed.
Merge pull request #15848 from sgrif/sg-remove-deprecation

Remove old deprecation warning
changelog entries for #15866 and #15848

/cc @sgrif
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