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 rails:update to the upgrading guides

[ci skip]
Add warning for `rescue_from Exception` and `rescue_from StandardError`

[ci skip]
Replace x.sort_by!.select! with x.select!.sort_by!

The latter has the same speed as the former in the worst case
and faster in general, because it is always better to sort less items.

Unfortunately, `routes.select!{...}.sort_by!{...}` is not possible here
because `select!` returns `nil`, so select! and sort! must be done
in two steps.
Return a null object from `AttributeSet#[]`
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.
Merge pull request #15856 from zuhao/refactor_activesupport_decoding_test

Use with_parse_json_times helper in tests.
Merge pull request #15850 from sgrif/sg-missing-tests

Add missing test cases for `attribute_method?`
Merge pull request #15849 from sgrif/sg-move-attributes

Move `attributes` to the `AttributeSet` object.
Merge pull request #15839 from sgrif/sg-attr-set-null

Return a null object from `AttributeSet#[]`
Create with_env helper for tests.
Merge pull request #15859 from zuhao/refactor_activesupport_caching_test

Create with_env helper for tests.
Merge pull request #15860 from aidanharan/docrails-assets-manifest-typo

[ci skip] Added missing colon in asset manifest JSON