RFR: JDK-8314489: Add javadoc index entries for java.lang.Math terms

[Joe Darcy]

Use of javadoc search index feature; I'll re-flow paragraphs before pushing once the wording is agreed to.

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Commit messages:
 - JDK-8314489: Add javadoc index entries for java.lang.Math terms

Changes: https://git.openjdk.org/jdk/pull/15322/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15322&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314489
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15322.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15322/head:pull/15322

PR: https://git.openjdk.org/jdk/pull/15322