RFR: 8278145: Javadoc for MemorySegment::set/MemorySegment::setAtIndex is missing throws tag

[Maurizio Cimadamore]

This patch adds missing `@throws` tags to the javadoc of `MemorySegment::set` and `MemorySegment::setAtIndex`.
These methods can fail with `UnsupportedOperationException` if the segment is read-only.

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Commit messages:
 - Add missing @throws tags

Changes: https://git.openjdk.java.net/jdk/pull/6668/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6668&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278145
  Stats: 17 lines in 1 file changed: 17 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6668.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6668/head:pull/6668

PR: https://git.openjdk.java.net/jdk/pull/6668