diff --git a/Documentation/pretty-options.txt b/Documentation/pretty-options.txt
index 27ddaf84a1..4f8376d681 100644
--- a/Documentation/pretty-options.txt
+++ b/Documentation/pretty-options.txt
@@ -36,9 +36,13 @@ people using 80-column terminals.
 	The commit objects record the encoding used for the log message
 	in their encoding header; this option can be used to tell the
 	command to re-code the commit log message in the encoding
-	preferred by the user.  For non plumbing commands this
-	defaults to UTF-8. Note that if an object claims to be encoded
-	in `X` and we are outputting in `X`, we will output the object
+	preferred by the user.
+	The encoding must be a system encoding supported by iconv(1),
+	otherwise this option will be ignored.
+	POSIX character maps used by iconv(1p) are not supported.
+	For non-plumbing commands this defaults to UTF-8.
+	Note that if an object claims to be encoded in `X`
+	and we are outputting in `X`, we shall output the object
 	verbatim; this means that invalid sequences in the original
 	commit may be copied to the output.