--- a/runtime/doc/spell.txt
+++ b/runtime/doc/spell.txt
@@ -1058,6 +1058,9 @@ this text to start with a "#" so that mi
 	SFX F 0 in   [^i]n      # Spion > Spionin  ~
 	SFX F 0 nen  in		# Bauerin > Bauerinnen ~
 
+However, to avoid lots of errors in affix files written for Myspell, you can
+add the IGNOREEXTRA flag.
+
 Apparently Myspell allows an affix name to appear more than once.  Since this
 might also be a mistake, Vim checks for an extra "S".  The affix files for
 Myspell that use this feature apparently have this flag.  Example:
@@ -1111,6 +1114,14 @@ Specifically, the affix flags can be use
 - CIRCUMFIX, as explained just below.
 
 
+IGNOREEXTRA						*spell-IGNOREEXTRA*
+
+Normally Vim gives an error for an extra field that does not start with '#'.
+This avoids errors going unnoticed.  However, some files created for Myspell
+or Hunspell may contain many entries with an extra field.  Use the IGNOREEXTRA
+flag to avoid lots of errors.
+
+
 CIRCUMFIX						*spell-CIRCUMFIX*
 
 The CIRCUMFIX flag means a prefix and suffix must be added at the same time.