RFR: JDK-8310575: no `@since` for StandardDoclet

[Jonathan Gibbons]

Please review a trivial update to provide a previously-missing `@since` tag for `StandardDoclet`

-------------

Commit messages:
 - JDK-8310575: no `@since` for StandardDoclet

Changes: https://git.openjdk.org/jdk/pull/14603/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14603&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310575
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14603.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14603/head:pull/14603

PR: https://git.openjdk.org/jdk/pull/14603