Fix a bug where NameError#name returns a qualified name in string

Ruby's original behaviour is that :

  * It only returns a const name, not a qualified aname
  * It returns a symbol, not a string
Merge pull request #15902 from yuki24/fix-name-error-bug

Fix a bug where NameError#name returns a qualified name in string
`preload` preserves readonly flag on associations. #15853

This is a partial fix for #15853. It only works when a `preload`
is issued and not an `eager_load`. I've added a skipped failing
test-case to keep in mind that we need to deal with `eager_load`.
synchronize Active Record deprecations / removals with 4.2 release notes

[ci skip]

/cc @chancancode @zzak
sync deprecations and removals with 4.2 release notes. [ci skip]

Noticed that there was no section about Action View and added the heading.
Notable changes need to be compiled.

/cc @chancancode @zzak
docs, fix BacktraceCleaner code example. Closes #15911. [ci skip]