RFR: [type-classes] 8378208: Improve documenation of textbook complex number prototypes

[Joe Darcy]

Doc cleanup.

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Commit messages:
 - JDK-8378208: Improve documenation of textbook complex number prototypes

Changes: https://git.openjdk.org/valhalla/pull/2133/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2133&range=00
  Stats: 34 lines in 2 files changed: 24 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/valhalla/pull/2133.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2133/head:pull/2133

PR: https://git.openjdk.org/valhalla/pull/2133