RFR: JDK-8329717 Missing `@since` tags in elements in DocumentationTool and Taglet

Fri Apr 5 00:06:16 UTC 2024

In this PR I added an `@since` tag to SNIPPET_PATH and isBlockTag() as they were added in later versions

- SNIPPET_PATH was added in JDK 18 [here](https://github.com/openjdk/jdk/commit/0fc47e99d20a1ee886df878f1302769bdd913aab#diff-8f73114f8b0d0d5229231541d5583382c8e8d33147e285f3d90ed8801ce9228bR192)

- isBlockTag() was added in JDK 15 [here](https://github.com/openjdk/jdk/commit/3c0e2b4e16d8fb2f96741bc8d4188aa47f58dd15#diff-3ee1b6e2a11b201a39ce3f2ac14ea9832900d8a3a581bf26577064173d0c9082R101)

This is similar to #18032

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Commit messages:
 - add `@since` to `SNIPPET_PATH` and `isBlockTag()`

Changes: https://git.openjdk.org/jdk/pull/18640/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18640&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329717
  Stats: 5 lines in 2 files changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18640.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18640/head:pull/18640

PR: https://git.openjdk.org/jdk/pull/18640