add "offset" prop to combobox and update doc

[qyiming]

Fixes #1451

Additional description

Same as menu-dropdown and any other component that uses dialog component, I would like to pass an "offset" prop down to dialog. This fix is needed to fix an SFX internal bug related to combobox.

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[qyiming]

The new "offset" is passed down to and handled by the dialog cmp, seems unnecessary to add a new test for it.

also re-generated the component doc for this change.

@interactivellama please review and I will at mention you on the internal duplicate bug. Thanks.

[interactivellama]

The offset prop is kind of a hack i dropdown. I'll follow up offline to see if there's an alternative way.

[interactivellama]

Please completely remove propType and place this comment in the source code above the line passing it into Dialog component.

[interactivellama]

Closing. Will re-open if #1454 doesn't work.