doc: add exception message for colliding property names

src/main/java/org/eclipse/yasson/internal/properties/MessageKeys.java

diff --git a/src/main/java/org/eclipse/yasson/internal/properties/MessageKeys.java b/src/main/java/org/eclipse/yasson/internal/properties/MessageKeys.java
index 2171f8b..1a6c0e0 100644
--- a/src/main/java/org/eclipse/yasson/internal/properties/MessageKeys.java
+++ b/src/main/java/org/eclipse/yasson/internal/properties/MessageKeys.java
@@ -327,7 +327,11 @@
     /**
      * Unknown visibility strategy.
      */
-    UNKNOWN_VISIBILITY_STRATEGY("unknownVisibilityStrategy");
+    UNKNOWN_VISIBILITY_STRATEGY("unknownVisibilityStrategy"),
+    /**
+     * Two properties or creator parameters collapse to the same key under CASE_INSENSITIVE naming strategy.
+     */
+    CASE_INSENSITIVE_COLLISION("caseInsensitiveCollision");
 
     /**
      * Message bundle key.

src/main/resources/yasson-messages.properties

diff --git a/src/main/resources/yasson-messages.properties b/src/main/resources/yasson-messages.properties
index 59b7d23..39dda13 100644
--- a/src/main/resources/yasson-messages.properties
+++ b/src/main/resources/yasson-messages.properties
@@ -90,3 +90,4 @@
 missingValuePropertyInAnnotation=Missing value property in Annotation {0}. Annotation will be ignored.
 numberIncompatibleValueTypeArray=Value type {0} is not a JsonNumber.
 numberIncompatibleValueTypeObject=Value type {0} at key {1} is not a JsonNumber.
+caseInsensitiveCollision=Properties ''{0}'' and ''{1}'' in class {2} collide to the same key ''{3}'' under the CASE_INSENSITIVE naming strategy.