The "EXIVAR" terminology has been replaced by "gen fields"
AKA "generic fields".

Exivar implies variable, but generic fields include more than
just variables, e.g. `object_id`.

Re-organize page docs

to adopt strict shareable rule.

* (basically) shareable objects only refer shareable objects
* (exception) shareable objects can refere unshareable objects
  but should not leak reference to unshareable objects to Ruby world

It does raise an exception rather than just issuing a warning:

```shell
$ docker run -e ALL_RUBY_SINCE=2.7 --rm rubylang/all-ruby ./all-ruby -We 'ENV.clone'
ruby-2.7.0
...
ruby-3.0.7
ruby-3.1.0-preview1 -e:1: warning: ENV.clone is deprecated; use ENV.to_h instead
...
ruby-3.2.0-preview1 -e:1: warning: ENV.clone is deprecated; use ENV.to_h instead
ruby-3.2.0-preview2 -e:1:in `clone': Cannot clone ENV, use ENV.to_h to get a copy of ENV as a hash (TypeError)
                    	from -e:1:in `<main>'
                exit 1
...
ruby-3.3.9          -e:1:in `clone': Cannot clone ENV, use ENV.to_h to get a copy of ENV as a hash (TypeError)
                    	from -e:1:in `<main>'
                exit 1
ruby-3.4.0-preview1 -e:1:in 'clone': Cannot clone ENV, use ENV.to_h to get a copy of ENV as a hash (TypeError)
                    	from -e:1:in '<main>'
                exit 1
...
ruby-3.5.0-preview1 -e:1:in 'clone': Cannot clone ENV, use ENV.to_h to get a copy of ENV as a hash (TypeError)
                    	from -e:1:in '<main>'
                exit 1

```