Remove `$LOADED_FEATURES` from examples in `lib/English.rb`

When the docs for `lib/English.rb` were first written [1],
`$LOADED_FEATURES` was an alias for `$"` defined in `lib/English.rb`.

Currently, however, `$LOADED_FEATURES` is defined in `load.c` along with
`$"` itself [2].

Update the documentation present in `lib/English.rb` to reflect that and
don't misguide the reader.

Also, add `$LOADED_FEATURES` to `doc/globals.rdoc`, which lists globals
defined by Ruby internals.

[1]: https://github.com/ruby/ruby/commit/f8c7b41165042b3d398a6a237aac9207e2856aea#diff-2c060ee58c8d1169e19a88d41b3f0259L3
[2]: https://github.com/ruby/ruby/blob/5cc1d57c3957ecabe02c5ad79dd75c523457187f/load.c#L1179