RFR: 1423: Clarfiy doc for /contributor command

[Erik Joelsson]

This patch improves the help text and the error messages for the /contributor command.

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Commit messages:
 - Remove unused import
 - Remove Note
 - Fix whitespace
 - SKARA-1423

Changes: https://git.openjdk.java.net/skara/pull/1314/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1314&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1423
  Stats: 13 lines in 1 file changed: 10 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1314.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1314/head:pull/1314

PR: https://git.openjdk.java.net/skara/pull/1314