RFR: JDK-8314477: Improve definition of "prototypical type"

[Joe Darcy]

Use an index tag to make the definition of "prototypical type" more findable.

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Commit messages:
 - JDK-8314477: Improve definition of "prototypical type"

Changes: https://git.openjdk.org/jdk/pull/15320/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15320&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314477
  Stats: 13 lines in 2 files changed: 6 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15320.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15320/head:pull/15320

PR: https://git.openjdk.org/jdk/pull/15320