📖 Docs for SyncPeriod: No sync from api-server.

[guettli]

There is a common misunderstanding about SyncPeriod. It does not sync between api-server and local cache.

This PR updates the docs to make this more clear.

Related: #3269 (comment)

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[alvaroaleman]

I am not a fan of changing this part of the paragaph, because the godoc goes from the big picture to the details/the why to the how, we are now adding parts of the how to the why.

Also it would IMHO be better to explain what does happen, rather than what does not.

WDYT about leaving this part be and changing L176 till the end of the godoc to:

	// SyncPeriod will locally trigger an artificial Update event with the same
	// object in both ObjectOld and ObjectNew for everything that is in the
	// cache.
	//
	// Predicates or Handlers that expect ObjectOld and ObjectNew to be different
	// (such as GenerationChangedPredicate) will filter out this event, preventing
	// it from triggering a reconciliation.