Document commentlink section in project.config file
Document commentlink section in project.config file

config-project-config.txt describes the sections that can appear in
project.config, but it missed to mention the commentlink section.

Change-Id: I7e53183e60179bfe051e17f91d3bcd19d172787b
Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>

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