RFR: 8298095: Refine implSpec for SegmentAllocator [v4]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Add snippet for MS::allocateArray methods
 - Merge branch 'master' into segallocjavadoc
 - Simplify snippets
 - Update after comments
 - Update array methods
 - Improve javadocs
 - Update javadocs for SegmentAllocator::allocate methods

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/14997/files
  - new: https://git.openjdk.org/jdk/pull/14997/files/66b1c8e5..c7544307

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14997&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14997&range=02-03

  Stats: 19765 lines in 753 files changed: 9812 ins; 4053 del; 5900 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