RFR: Moved backport text to backport section

[Jesper Wilhelmsson]

Moved "How to fix an incorrect backport creation in JBS" to the backport section. The section is unchanged, only moved. Also some minor improvements (in my mind) to the "JBS Labels" section.

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Commit messages:
 - Moved backport text to backport section

Changes: https://git.openjdk.java.net/guide/pull/78/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=78&range=00
  Stats: 75 lines in 1 file changed: 38 ins; 34 del; 3 mod
  Patch: https://git.openjdk.java.net/guide/pull/78.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/78/head:pull/78

PR: https://git.openjdk.java.net/guide/pull/78