RFR: 1725: Clarify "no known role" error message

[Zhao Song]

A user reported that when people review PRs, the bot adds a list of reviewers into description. If a user is not registered in OpenJDK Census, a note is added: 

(no known github.com user name / role) 

What it actually means is "no known OpenJDK user name / role".

Github.com is being used here because we are utilizing the name of the namespace. However, the purpose of the namespace is to map users from this namespace to either openjdk or internal. Actually, we couldn't find the user in openjdk or internal, not in the namespace. The census repository name can indicate whether it is openjdk or internal.

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Commit messages:
 - SKARA-1725

Changes: https://git.openjdk.org/skara/pull/1459/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1459&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1725
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1459.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1459/head:pull/1459

PR: https://git.openjdk.org/skara/pull/1459