git-svn: clarify the "Ignoring error from SVN" piece

I've heard of several users puzzled by this, and it sometimes it
appears as if git-svn is doing nothing on slower connections and
larger repositories.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-svn.perl

@@ -3488,6 +3488,10 @@ sub skip_unknown_revs {
 			warn "W: Ignoring error from SVN, path probably ",
 			     "does not exist: ($errno): ",
 			     $err->expanded_message,"\n";
+			warn "W: Do not be alarmed at the above message ",
+			     "git-svn is just searching aggressively for ",
+			     "old history.\n",
+			     "This may take a while on large repositories\n";
 			$ignored_err{$err_key} = 1;
 		}
 		return;