DOC: Reword description about patching in preview mode

[david-cortes-intel]

Description

This PR rewords the documentation about patching in preview mode to make it less ambiguous.

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[icfaust]

Are we sure that the sklearnex-only estimators don't get patched-in? https://github.com/uxlfoundation/scikit-learn-intelex/blob/main/sklearnex/dispatcher.py#L62 It takes the mapping without preview, and then adds the preview estimators on top, and for the sklearnex-only estimators, if it didn't patch in, we would not have sklearn conformance deselections for those estimators (mostly for test_common.py). Maybe I misunderstood? Let me know. This PR should be quick to approve once we iron this out (figure out what I got wrong here).

[david-cortes-intel]

Yes, looks like patching still makes them importable from sklearn. Modified the docs here and in the section about extension estimators.

[icfaust]

/azp run Docs

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[icfaust]

Somehow the Docs CI has failed, and started a re-run. Can be merged if it comes out as passing.