Comments enabled for registered users

In response to a developer's question why page editing was permitted but not comment submitting, comments were enabled. A notice (visible to logged-in users) was added to each page bottom to request comments be about the documentation itself and not the documented feature, etc.

Comments weren't enabled before because of the concern that end users might ask support questions at the documentation page instead of in the forums. We can monitor how things go and make appropriate adjustments.