Fix encode/decode method doc of str, bytes, bytearray types

 * Specify the default encoding: write 'utf-8' instead of
   sys.getdefaultencoding(), because the default encoding is now constant
 * Specify the default errors value
diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index 283102a..1a50ce3 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -2465,10 +2465,10 @@
 }
 
 PyDoc_STRVAR(decode_doc,
-"B.decode([encoding[, errors]]) -> str\n\
+"B.decode([encoding='utf-8'[, errors='strict']]) -> str\n\
 \n\
-Decode B using the codec registered for encoding. encoding defaults\n\
-to the default encoding. errors may be given to set a different error\n\
+Decode B using the codec registered for encoding. Default encoding\n\
+is 'utf-8'. errors may be given to set a different error\n\
 handling scheme.  Default is 'strict' meaning that encoding errors raise\n\
 a UnicodeDecodeError.  Other possible values are 'ignore' and 'replace'\n\
 as well as any other name registered with codecs.register_error that is\n\