RFR: Backporting related changes

[Jesper Wilhelmsson]

Added a section on backporting multiple related changes.
Replaced "push" with "integrate" in several places since developers can't push to the OpenJDK repositories.
Removed the JBS label vthreads that has been retired and is no longer in use in JBS.

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

Commit messages:
 - Backporting related changes

Changes: https://git.openjdk.org/guide/pull/129/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=129&range=00
  Stats: 28 lines in 3 files changed: 10 ins; 6 del; 12 mod
  Patch: https://git.openjdk.org/guide/pull/129.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/129/head:pull/129

PR: https://git.openjdk.org/guide/pull/129