RFR: 8333886: Explicitly specify that asSlice and reinterpret return a memory segment backed by the same region of memory.

[Per Minborg]

This PR proposes to explicitly state that returned segments form the `asSlice` and `reinterpret` method share memory regions with `this` memory segment.

Note: The term "subset"  means a true subset or the same set.

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

Commit messages:
 - Add segments share the same backing store in docs

Changes: https://git.openjdk.org/jdk/pull/19633/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19633&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333886
  Stats: 21 lines in 1 file changed: 21 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19633.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19633/head:pull/19633

PR: https://git.openjdk.org/jdk/pull/19633