More intuitive description for never/disable option for explorer.enableUndo

[rebornix]

Testing #141351

It's not clear what disable/never means for explorer.enableUndo. allow/warn are talking about when the explorer will prompt to warn users but never disable this feature completely. I didn't get that when the first time read does not anticipate in undo events, maybe we make it clear that undo/redo will no longer work in explorer when disabled.

[JacksonKearl]

Moved to Undo will be disabled in the explorer.

[JacksonKearl]

Actually split this into two settings, so explorer.enableUndo is a boolean controlling whether the explorer does undo at all, then explorer.confirmUndo is an enum configuring how frequently to prompt.