HP3PAR Block Storage extra-specs key fix

This fix will cover the documentation impact related to the scoping
of custom extra-specs changes fixed in bug: 1194289

https://bugs.launchpad.net/cinder/+bug/1194289

Cherry picked from https://review.openstack.org/34465

Change-Id: I2984278655afa7ded2fd4100b374f8406f4e3054
Merge "HP3PAR Block Storage extra-specs key fix" into stable/grizzly