refactor the naming within plugin new generator

Having a method called `full?`, which checks on :full and :mountable
is very confusing. I renamed `full?` to `engine?` and created a `full?`
method that only checks the `:full` option
Relation.where with no args can be chained with not, like, and not_like

examples:

  Model.where.not field: nil
  #=> "SELECT * FROM models WHERE field IS NOT NULL

  Model.where.like name: 'Jeremy%'
  #=> "SELECT * FROM models WHERE name LIKE 'Jeremy%'

this feature was originally suggested by Jeremy Kemper https://github.com/rails/rails/pull/5950#issuecomment-5591330

Closes #5950
Update documentation for javascript_include_tag [ci skip]
remove unneeded Examples tag [ci skip]
Improves documentation on favicon_link_tag
Removes :nodoc: label [ci skip]
nodoc AS::Testing::Pending because is deprecated [ci skip]
Remove :nodoc from SanitizeHelper module [ci skip]
nodoc AD & AV railties classes
Removes unneeded 'Example' tags and whitespaces [ci skip]
Removes :nodoc: tag on AtomFeedHelper [ci skip]
Improves documentation of Capture helper [ci skip]
Add documentation to TransactionIsolationError [ci skip]
Remove trailing whitespaces
Revert "Remove trailing whitespaces"

This reverts commit 90c887fa7d0c454b7533e208daefc342dea4d5f3.
Migration Guide: General fix-up

This introduces a bunch of editing/re-writing to the migrations guide.
There were a bunch of small changes, and a few larger ones:

* mysql does support transactions.
* Add a section about db/seeds.rb
* Largely re-wrote the first few sections
Cleans documentation from Helpers [ci skip]
Fixes wrong typo on FormHelper [ci skip]
update AR::Schema docs [ci skip]
Add documentation to AssetTagHelper#image_alt [ci skip]