RFR: 8298095: Refine implSpec for SegmentAllocator [v5]

[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:

  Simplify java snippets

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14997/files
  - new: https://git.openjdk.org/jdk/pull/14997/files/c7544307..f0f80b85

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14997&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14997&range=03-04

  Stats: 14 lines in 1 file changed: 0 ins; 7 del; 7 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