Document the new -y option to choose which font in a TTC to decompile.

git-svn-id: svn://svn.code.sf.net/p/fonttools/code/trunk@571 4cde692c-a291-49d1-8350-778aa11640f8
diff --git a/Doc/ttx.1 b/Doc/ttx.1
index 5c3017a..2b64c47 100644
--- a/Doc/ttx.1
+++ b/Doc/ttx.1
@@ -119,8 +119,11 @@
 assembly.  This saves some time and results in smaller
 .Tn TTX
 files.
-.It Fl e
-Don't ignore decompilation errors, but show a full traceback and abort.
+.It Fl y Ar n
+When decompiling a TrueType Collection (TTC) file,
+decompile font number
+.Ar n ,
+starting from 0.
 .El
 .Ss "Compilation options"
 The following options control the process of compiling