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Fix T87983: Motion tracker button tooltips identical

The tooltips for the tracker buttons were all equal (because they all
use the same operator).

This adds the `get_description` operator callback to the
`CLIP_OT_track_markers` operator and returns a more descriptive
tooltip for the different buttons.

Reviewed By: mont29

Maniphest Tasks: T87983

Differential Revision: https://developer.blender.org/D11145