[ObjC] Update comments about common failure causes.

PiperOrigin-RevId: 509914968

objectivec/GPBMessage.h

diff --git a/objectivec/GPBMessage.h b/objectivec/GPBMessage.h
index 5b7ce0c..d9fc898 100644
--- a/objectivec/GPBMessage.h
+++ b/objectivec/GPBMessage.h
@@ -293,6 +293,10 @@
  *
  * @note This can raise the GPBCodedOutputStreamException_* exceptions.
  *
+ * @note The most common cause of this failing is from one thread calling this
+ *       while another thread has a reference to this message or a message used
+ *       within a field and that other thread mutating the message while this
+ *       serialization is taking place.
  **/
 - (void)writeToCodedOutputStream:(GPBCodedOutputStream *)output;
 
@@ -302,6 +306,11 @@
  * @param output The output stream into which to write the message.
  *
  * @note This can raise the GPBCodedOutputStreamException_* exceptions.
+ *
+ * @note The most common cause of this failing is from one thread calling this
+ *       while another thread has a reference to this message or a message used
+ *       within a field and that other thread mutating the message while this
+ *       serialization is taking place.
  **/
 - (void)writeToOutputStream:(NSOutputStream *)output;
 
@@ -312,6 +321,11 @@
  * @param output The coded output stream into which to write the message.
  *
  * @note This can raise the GPBCodedOutputStreamException_* exceptions.
+ *
+ * @note The most common cause of this failing is from one thread calling this
+ *       while another thread has a reference to this message or a message used
+ *       within a field and that other thread mutating the message while this
+ *       serialization is taking place.
  **/
 - (void)writeDelimitedToCodedOutputStream:(GPBCodedOutputStream *)output;
 
@@ -322,6 +336,11 @@
  * @param output The output stream into which to write the message.
  *
  * @note This can raise the GPBCodedOutputStreamException_* exceptions.
+ *
+ * @note The most common cause of this failing is from one thread calling this
+ *       while another thread has a reference to this message or a message used
+ *       within a field and that other thread mutating the message while this
+ *       serialization is taking place.
  **/
 - (void)writeDelimitedToOutputStream:(NSOutputStream *)output;
 
@@ -336,6 +355,11 @@
  * @note In DEBUG ONLY, the message is also checked for all required field,
  *       if one is missing, nil will be returned.
  *
+ * @note The most common cause of this failing is from one thread calling this
+ *       while another thread has a reference to this message or a message used
+ *       within a field and that other thread mutating the message while this
+ *       serialization is taking place.
+ *
  * @return The binary representation of the message.
  **/
 - (nullable NSData *)data;
@@ -347,6 +371,11 @@
  * @note This value is not cached, so if you are using it repeatedly, it is
  *       recommended to keep a local copy.
  *
+ * @note The most common cause of this failing is from one thread calling this
+ *       while another thread has a reference to this message or a message used
+ *       within a field and that other thread mutating the message while this
+ *       serialization is taking place.
+ *
  * @return The binary representation of the size along with the message.
  **/
 - (NSData *)delimitedData;