GH-117759: Update GC docs for incremental collection

[markshannon]

• Issue: Document the new incremental GC #117759

[iritkatriel]

Suggested change

	In order to limit the time each garbage collection takes, the GC implementation
	In order to bound the length of each garbage collection pause, the GC implementation

[iritkatriel]

Suggested change

	The time taken to scan the young generation can be controlled by controlling the
	size of the young, but the size of the old generation cannot be controlled.
	The time taken to scan the young generation can be controlled by controlling its
	size, but the size of the old generation cannot be controlled.

[iritkatriel]

what does "by a single collection" mean here?