RFR: Cleaning up headers and examples

[Jesper Wilhelmsson]

* Make all headers follow the same style
* Make code examples use ~~~ instead of four space indentation
* Fix email adresses to be actual adresses
* Add link to jtreg FAQ
* Add comment about the name space for noreg-labels

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Commit messages:
 - Cleaning up headers and examples, and clarifying some wording.

Changes: https://git.openjdk.org/guide/pull/90/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=90&range=00
  Stats: 91 lines in 6 files changed: 29 ins; 0 del; 62 mod
  Patch: https://git.openjdk.org/guide/pull/90.diff
  Fetch: git fetch https://git.openjdk.org/guide pull/90/head:pull/90

PR: https://git.openjdk.org/guide/pull/90