RFR: 8298095: Refine implSpec for SegmentAllocator [v3]

[Per Minborg]

> This PR suggests refining the `@implSpec` for the SegmentAllocator::allocate methods as well as clarifying the docs a bit more. Also, a local variable is renamed.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Update after comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14997/files
  - new: https://git.openjdk.org/jdk/pull/14997/files/e4d0a4bc..66b1c8e5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14997&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14997&range=01-02

  Stats: 139 lines in 1 file changed: 39 ins; 5 del; 95 mod
  Patch: https://git.openjdk.org/jdk/pull/14997.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14997/head:pull/14997

PR: https://git.openjdk.org/jdk/pull/14997