RFR: 8255020: Minor updates to docs jdk.incubator.vector

[Paul Sandoz]

Minor updates, with no specification changes, to the documentation of Vector API.

The compilation of the Vector module was updated to turn on doclint errors for >= protected documentation.

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Commit messages:
 - 8255020:Minor updates to docs jdk.incubator.vector

Changes: https://git.openjdk.java.net/jdk/pull/746/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=746&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255020
  Stats: 24 lines in 4 files changed: 15 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/746.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/746/head:pull/746

PR: https://git.openjdk.java.net/jdk/pull/746