`default_enforce_utf8` belongs to `config.action_view`
Update info about `:skip_enforcing_utf8` since we can change default
behavior via `config.action_controller.default_enforce_utf8`
Related to rails#32125
This commit is the next work after rails#33523.
Also, this commit removes mention about hidden `utf8` input. Since
form helpers don't generate this input by default since rails#32125.
Note that I also had created PR rails#31972 with improvements to
"Action View Form Helpers" guide, but I'll rebase it after merging the