fix: document reason for getFormattedMessage call

Adds comments to explain the side effects of the `getFormattedMessage` calls.

Author: ppkarwasz

log4j-api/src/main/java/org/apache/logging/log4j/message/ReusableObjectMessage.java

@@ -129,6 +129,8 @@ public <S> void forEachParameter(final ParameterConsumer<S> action, final S stat
     @Override
     public Message memento() {
         Message message = new ObjectMessage(obj);
+        // Since `toString()` methods are not always pure functions and might depend on the thread and other context
+        // values, we format the message and cache the result.
         message.getFormattedMessage();
         return message;
     }

log4j-api/src/main/java/org/apache/logging/log4j/message/ReusableParameterizedMessage.java

@@ -115,6 +115,8 @@ public <S> void forEachParameter(final ParameterConsumer<S> action, final S stat
     @Override
     public Message memento() {
         Message message = new ParameterizedMessage(messagePattern, getTrimmedParams());
+        // Since `toString()` methods are not always pure functions and might depend on the thread and other context
+        // values, we format the message and cache the result.
         message.getFormattedMessage();
         return message;
     }

log4j-api/src/main/java/org/apache/logging/log4j/message/ReusableSimpleMessage.java

@@ -86,6 +86,8 @@ public <S> void forEachParameter(final ParameterConsumer<S> action, final S stat
     @Override
     public Message memento() {
         SimpleMessage message = new SimpleMessage(charSequence);
+        // Since `toString()` methods are not always pure functions and might depend on the thread and other context
+        // values, we format the message and cache the result.
         message.getFormattedMessage();
         return message;
     }