Added complete Doc Conventions in common guide.

Not backported since it does not exist in havana:
	doc/common/section_keystone-concepts-domain-management.xml

backport: havana
Change-Id: I6851ef705247d619e78fb63cbcdf1ce804a548f1
Partial-Bug: #1121866
(cherry picked from commit 201f5c66390d8c614d1e487d2e57d36149efeb50)