RFR: [type-classes] 8378210: Improve documenation of PolynomialDouble prototype

[Joe Darcy]

Doc improvements.

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Commit messages:
 - JDK-8378210: Improve documenation of PolynomialDouble prototype

Changes: https://git.openjdk.org/valhalla/pull/2134/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2134&range=00
  Stats: 11 lines in 1 file changed: 6 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/2134.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2134/head:pull/2134

PR: https://git.openjdk.org/valhalla/pull/2134