RFR: JDK-8329717 Missing `@since` tags in elements in DocumentationTool and Taglet [v2]

[Nizar Benalla]

> 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

Nizar Benalla has updated the pull request incrementally with one additional commit since the last revision:

  add blank line

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18640/files
  - new: https://git.openjdk.org/jdk/pull/18640/files/0ffee115..0c5a300d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18640&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18640&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 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