RFR: 1423: Clarfiy doc for /contributor command [v2]

[Erik Joelsson]

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

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Fix failing test

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1314/files
  - new: https://git.openjdk.java.net/skara/pull/1314/files/28f617bb..5824a75f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1314&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1314&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 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