[foreign-memaccess] RFR: larify exception behaviour for MemorySegment::toByteArray when the segment does not feature the READ access mode.

[Chris Hegarty]

Clarify exception behaviour for MemorySegment::toByteArray when the segment does not feature the READ access mode.

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Commit messages:
 - typo
 - Clarify exception behaviour for MemorySegment::toByteArray when the segment does not feature the READ access mode.

Changes: https://git.openjdk.java.net/panama-foreign/pull/178/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/178/webrev.00
  Stats: 18 lines in 2 files changed: 16 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/178.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/178/head:pull/178

PR: https://git.openjdk.java.net/panama-foreign/pull/178