[PDS-134006] SweepStrategy.THOROUGH_MIGRATION docs

[jeremyk-91]

Goals (and why):

• Actually have people know how to use THOROUGH_MIGRATION mode, which hadn't been documented.

Implementation Description (bullets):

• Add docs

Testing (What was existing testing like? What have you done to improve it?):
Docs

Concerns (what feedback would you like?):

• Are the processes correct?

Where should we start reviewing?: the one file

Priority (whenever / two weeks / yesterday): this week (assuming you see this on Monday)

[gmaretic]

LGTM!