RFR: 8323634: Shenandoah: Document behavior of EvacOOM protocol [v3]

[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:

  Initialize evacuation state

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17385/files
  - new: https://git.openjdk.org/jdk/pull/17385/files/8f31b884..23e0d4b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17385&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17385&range=01-02

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 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