[foreign-memaccess] RFR: 8247924: Improve javadoc of Foreign Memory Access API (part one)

[Maurizio Cimadamore]

This patch clarifies the role of `MemorySegment::close`, and also mentions the fact that external memory sources
(arrays/buffers) can be aliased, and that extra care must be taken by clients wanting to share such segments.

I also fixed the typos in restricted methods which we also fixed in foreign-abi.

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Commit messages:
 - Fix javadoc for close()
 - Clarify javadoc w.r.t close and aliasing of existing memory sources.

Changes: https://git.openjdk.java.net/panama-foreign/pull/209/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/209/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247924
  Stats: 16 lines in 1 file changed: 10 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/209.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/209/head:pull/209

PR: https://git.openjdk.java.net/panama-foreign/pull/209