RFR: 8321935: Define the term 'standard doclet'

[Jonathan Gibbons]

Please review this doc-only fix to indicate that the `StandardDoclet` class is the one used by default by the _javadoc_ tool.

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Commit messages:
 - JDK-8321935: Define the term 'standard doclet'

Changes: https://git.openjdk.org/jdk/pull/20952/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20952&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321935
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20952.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20952/head:pull/20952

PR: https://git.openjdk.org/jdk/pull/20952