log: document --encoding behavior on iconv() failure

We already note that we may produce invalid output when we skip calling
iconv() altogether. But we may also do so if iconv() fails, and we have
no good alternative. Let's document this to avoid surprising users.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-options.txt

@@ -40,7 +40,9 @@ people using 80-column terminals.
 	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
 	verbatim; this means that invalid sequences in the original
-	commit may be copied to the output.
+	commit may be copied to the output. Likewise, if iconv(3) fails
+	to convert the commit, we will quietly output the original
+	object verbatim.
 
 --expand-tabs=<n>::
 --expand-tabs::