document multiple recipients in XML configs

Adds an example on how to configure multiple recipients in Monolog
for sending log entries as e-mails.
feature #4140 [Cookbook][Logging] document multiple recipients in XML configs (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Cookbook][Logging] document multiple recipients in XML configs

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/MonologBundle#91)
| Applies to    | all
| Fixed tickets | #3940, #4135

Commits
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37ab23d document multiple recipients in XML configs