Fix error and add note about lossy issues

objectivec/GPBWellKnownTypes.h

@@ -54,7 +54,12 @@ NS_ASSUME_NONNULL_BEGIN
 /** The NSDate representation of this GPBTimestamp. */
 @property(nonatomic, readwrite, strong) NSDate *date;
 
-/** The NSTimeInterval representation of this GPBTimestamp. */
+/**
+ * The NSTimeInterval representation of this GPBTimestamp.
+ *
+ * @note: Not all second/nanos combinations can be represented in a
+ * NSTimeInterval, so getting this could be a lossy transform.
+ **/
 @property(nonatomic, readwrite) NSTimeInterval timeIntervalSince1970;
 
 /**
@@ -82,7 +87,12 @@ NS_ASSUME_NONNULL_BEGIN
  **/
 @interface GPBDuration (GBPWellKnownTypes)
 
-/** The NSTimeInterval representation of this GPBTimestamp. */
+/**
+ * The NSTimeInterval representation of this GPBDuration.
+ *
+ * @note: Not all second/nanos combinations can be represented in a
+ * NSTimeInterval, so getting this could be a lossy transform.
+ **/
 @property(nonatomic, readwrite) NSTimeInterval timeIntervalSince1970;
 
 /**
@@ -90,7 +100,7 @@ NS_ASSUME_NONNULL_BEGIN
  *
  * @param timeIntervalSince1970 Time interval to configure the GPBDuration with.
  *
- * @return A newly initialized GPBTimestamp.
+ * @return A newly initialized GPBDuration.
  **/
 - (instancetype)initWithTimeIntervalSince1970:(NSTimeInterval)timeIntervalSince1970;