I keep a copy of the cas project locally. I use it to search for properties, log messages and classes.
$ grep -rln cifsServicePrincipal # leave out the first letter since it may be lower or upper case in files
api/cas-server-core-api-configuration-model/src/main/java/org/apereo/cas/configuration/model/support/spnego/SpnegoAuthenticationProperties.java
support/cas-server-support-spnego/src/test/java/org/apereo/cas/support/spnego/authentication/handler/support/JcifsConfigTests.java
support/cas-server-support-spnego/src/main/java/org/apereo/cas/config/SpnegoConfiguration.java
support/cas-server-support-spnego/src/main/java/org/apereo/cas/support/spnego/authentication/handler/support/JcifsConfig.java
The first hit has the property you were asking about. You can then open that file locally or on github to see what other properties are at that level.
The next search you can perform going up the property hierarchy,
$ grep -rln SpnegoAuthenticationProperties
api/cas-server-core-api-configuration-model/src/main/java/org/apereo/cas/configuration/model/support/spnego/SpnegoProperties.java
api/cas-server-core-api-configuration-model/src/main/java/org/apereo/cas/configuration/model/support/spnego/SpnegoAuthenticationProperties.java
The first one has the properties field and it is a list.
You may need to turn on debug logging to see which properties are identified as incorrect. There might be a way to tell spring to fail if a property is incorrect.
Ray