Add revocable memory and reserved memory pool docs to spill
Add revocable memory and reserved memory pool docs to spill

Cherry-pick of https://github.com/prestosql/presto/pull/285 and
https://github.com/prestosql/presto/pull/1226

Co-authored-by: praveenkrishna <cc46d21fb853bd445ccce50fc3e94e715ff9d326@tutanota.com>
Co-authored-by: Ariel Weisberg <fb62a6c026c4657e72460f12749c7f83abda42dd@fb.com>

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