RFR: Add notes on avoiding spurious introduction of labels.

[Stuart Marks]

Include discussion of using label autocomplete to ensure that an existing label is used.

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Commit messages:
 - Add notes on avoiding spurious introduction of labels.

Changes: https://git.openjdk.java.net/guide/pull/31/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=31&range=00
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/guide/pull/31.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/31/head:pull/31

PR: https://git.openjdk.java.net/guide/pull/31