RFR: 8298095: Refine implSpec for SegmentAllocator

[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.

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Commit messages:
 - Improve javadocs
 - Update javadocs for SegmentAllocator::allocate methods

Changes: https://git.openjdk.org/jdk/pull/14997/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14997&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298095
  Stats: 105 lines in 1 file changed: 48 ins; 8 del; 49 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