RFR: 8323634: Shenandoah: Document behavior of EvacOOM protocol [v4]

[Kelvin Nilsen]

> The protocol for handling OOM during evacuation is subtle and critical for correct operation.  This PR does NOT change behavior.  It provided improved documentation of existing behavior.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Add clarification to multiple comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17385/files
  - new: https://git.openjdk.org/jdk/pull/17385/files/23e0d4b4..d1d1c1c6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17385&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17385&range=02-03

  Stats: 35 lines in 2 files changed: 25 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/17385.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17385/head:pull/17385

PR: https://git.openjdk.org/jdk/pull/17385