RFR: 8342653: Fix minor doc issues in AnnotatedElement

[Joe Darcy]

Fix typo and remove redundant `@since` tags.

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Commit messages:
 - JDK-8342653: Fix minor doc issues in AnnotatedElement

Changes: https://git.openjdk.org/jdk/pull/21597/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21597&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342653
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21597.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21597/head:pull/21597

PR: https://git.openjdk.org/jdk/pull/21597