GenEx: Add COMPATIBLE_INTERFACE_ implementation comments

Source/cmGeneratorExpressionNode.cxx

diff --git a/Source/cmGeneratorExpressionNode.cxx b/Source/cmGeneratorExpressionNode.cxx
index 94803da..e3cb497 100644
--- a/Source/cmGeneratorExpressionNode.cxx
+++ b/Source/cmGeneratorExpressionNode.cxx
@@ -2950,6 +2950,19 @@
       return std::string();
     }
 
+    // Properties named by COMPATIBLE_INTERFACE_ properties combine over
+    // the transitive link closure as a single order-independent value.
+    // Imported targets do not themselves have a defined value for these
+    // properties, but they can contribute to the value of a non-imported
+    // dependent.
+    //
+    // For COMPATIBLE_INTERFACE_{BOOL,STRING}:
+    // * If set on this target, use the value directly.  It is checked
+    //   elsewhere for consistency over the transitive link closure.
+    // * If not set on this target, compute the value from the closure.
+    //
+    // For COMPATIBLE_INTERFACE_NUMBER_{MAX,MIN} we always compute the value
+    // from this target and the transitive link closure to get the max or min.
     if (!haveProp && !target->IsImported() &&
         target->GetType() != cmStateEnums::INTERFACE_LIBRARY) {
       if (target->IsLinkInterfaceDependentBoolProperty(propertyName,
@@ -2989,7 +3002,6 @@
         return propContent ? propContent : "";
       }
     }
-
     if (!target->IsImported() && dagCheckerParent &&
         !dagCheckerParent->EvaluatingLinkLibraries()) {
       if (target->IsLinkInterfaceDependentNumberMinProperty(propertyName,
@@ -3010,6 +3022,8 @@
       }
     }
 
+    // Some properties, such as usage requirements, combine over the
+    // transitive link closure as an ordered list.
     if (!interfacePropertyName.empty()) {
       result = cmGeneratorExpression::StripEmptyListElements(
         this->EvaluateDependentExpression(result, context->LG, context, target,