Rename the enum_prefix and enum_suffix options to _prefix and _suffix

The note regarding the `_prefix` and `_suffix` options is no longer
useful since they were renamed specifically for this purpose.

Also the given example doesn't show what these options enable and in
which case they are really useful (when there are conflicting values
for instance).

Refs #20999.

[Godfrey Chan & Robin Dupret]

… skip]

`:enum_prefix` and `:enum_suffix` was changed to `:_prefix` and `:_suffix`
in rails#20999.

Unlike other features built on Attribute API, reserved options for
`enum` has leading `_`.

* `_prefix`/`_suffix`: rails#19813, rails#20999
* `_scopes`: rails#34605
* `_default`: rails#39820

That is due to `enum` takes one hash argument only, which contains both
enum definitions and reserved options.

I propose new syntax for `enum` to avoid leading `_` from reserved
options, by allowing `enum(attr_name, ..., **options)` more Attribute
API like syntax.

Before:

```ruby
class Book < ActiveRecord::Base
  enum status: [ :proposed, :written ], _prefix: true, _scopes: false
  enum cover: [ :hard, :soft ], _suffix: true, _default: :hard
end
```

After:

```ruby
class Book < ActiveRecord::Base
  enum :status, [ :proposed, :written ], prefix: true, scopes: false
  enum :cover, [ :hard, :soft ], suffix: true, default: :hard
end
```