docstring update: 'keyschema' validation rule.
docstring update: 'keyschema' validation rule.

brettatoms/cerberus,dkellner/cerberus,pyeve/cerberus,funkyfuture/cerberus,calve/cerberus,pyeve/cerberus,nicolaiarocci/cerberus,calve/cerberus,MacHu-GWU/cerberus-sanhe,funkyfuture/cerberus,MacHu-GWU/cerberus-sanhe,nicolaiarocci/cerberus,dkellner/cerberus