RFR: 8300792: Refactor examples in java.net.http to use @snippet [v4]

[Darragh Clarke]

> Refactored instances of `@code` to instead use `@snippet`.
> 
> I also spent some time looking into using external snippets to reference the existing files `JavadocExamples` and `WebSocketExample` though ultimately thought it was better to keep the docs pretty much as they had been with only a few small changes to formatting.

Darragh Clarke has updated the pull request incrementally with one additional commit since the last revision:

  missed one set of examples

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12692/files
  - new: https://git.openjdk.org/jdk/pull/12692/files/19944c7c..d8f58fce

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12692&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12692&range=02-03

  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12692.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12692/head:pull/12692

PR: https://git.openjdk.org/jdk/pull/12692