Documentation: Add backlog/group information to CM
Documentation: Add backlog/group information to CM

Persist what needs to be persisted in the Community Manager
documentation from [1], i.e., the key parts of [2] and [3].

Insert these few paragraphs right after mentioning the CM role's list of
tasks, as they complement the latter.

[1] https://www.gerritcodereview.com/2019-06-28-cm-announce.html
[2] https://www.gerritcodereview.com/2019-06-28-cm-announce.html#community-backlog
[3] https://www.gerritcodereview.com/2019-06-28-cm-announce.html#confidential-group

Feature: Issue 10978
Change-Id: I4e993a14c8ca52baf1554fa4bb80e6492a3f1c59

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