[vectorIntrinsics] RFR: Correct specification for length-invariance.

[Paul Sandoz]

Minor clarifications to the specification.

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Commit messages:
 - Correct specification for length-invariance.

Changes: https://git.openjdk.java.net/panama-vector/pull/76/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=76&range=00
  Stats: 7 lines in 1 file changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/76.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/76/head:pull/76

PR: https://git.openjdk.java.net/panama-vector/pull/76