Fix V3 credential behavior, documentation

Fixing V3 credential behavior so that contradicting parameter combinations
do not result in unpredictable behavior.  Updating accounts.yaml.sample
file to reference the correct location of the credentials
classes and to describe the updated behavior of Identity V3 attributes.

Change-Id: I29efe778afcb1e4a55dffd6a8ed8212d62a4dd15
diff --git a/etc/accounts.yaml.sample b/etc/accounts.yaml.sample
index decc659..3dbed79 100644
--- a/etc/accounts.yaml.sample
+++ b/etc/accounts.yaml.sample
@@ -3,7 +3,25 @@
 # This is required to provide isolation between test for running in parallel
 #
 # Valid fields for credentials are defined in the descendants of
-# auth.Credentials - see KeystoneV[2|3]Credentials.CONF_ATTRIBUTES
+# lib.auth.Credentials - see KeystoneV[2|3]Credentials.ATTRIBUTES
+#
+# The fields in KeystoneV3Credentials behave as follows:
+#
+# tenant_[id|name] also sets project_[id|name].
+#
+# project_[id|name] also sets tenant_[id|name].
+#
+# Providing distinct values for both tenant_[id|name] and project_[id|name]
+# will result in an InvalidCredentials exception.
+#
+# The value of project_domain_[id|name] is used for user_domain_[id|name] if
+# the latter is not specified.
+#
+# The value of user_domain_[id|name] is used for project_domain_[id|name] if
+# the latter is not specified.
+#
+# The value of domain_[id|name] is used for project_domain_[id|name] if not
+# specified and user_domain_[id|name] if not specified.
 
 - username: 'user_1'
   tenant_name: 'test_tenant_1'