RFR: 8266666: Implementation for snippets [v3]

[Pavel Rappo]

> This PR implements JEP 413 "Code Snippets in Java API Documentation", which hasn't been yet proposed to target JDK 18. The PR starts as a squashed merge of the https://github.com/openjdk/jdk-sandbox/tree/jdk.javadoc/snippets branch.

Pavel Rappo has updated the pull request incrementally with one additional commit since the last revision:

  Improve comments
  
  This commit adds trivial commentary to some of the Java declarations. This commit also adds the "This is NOT part of any supported API..." disclaimer.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4795/files
  - new: https://git.openjdk.java.net/jdk/pull/4795/files/c4e5b79c..43009031

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=01-02

  Stats: 145 lines in 12 files changed: 139 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4795.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4795/head:pull/4795

PR: https://git.openjdk.java.net/jdk/pull/4795