Clarify things around the coding cookie.

Serhiy Storchaka: "Coding cookie must be in comment, only the first
occurrence in the line must be taken to account (here is a bug in
CPython), encoding name must be ASCII, and there must not be any
Python statement on the line that contains the encoding
declaration."

pep-0263.txt

@@ -61,11 +61,13 @@ Defining the Encoding
           # vim: set fileencoding=<encoding name> :
 
     More precisely, the first or second line must match the regular
-    expression "coding[:=]\s*([-\w.]+)". The first group of this
+    expression "^[ \t\v]*#.*?coding[:=][ \t]*([-.a-zA-Z0-9]+)".
+    The first group of this
     expression is then interpreted as encoding name. If the encoding
     is unknown to Python, an error is raised during compilation. There
     must not be any Python statement on the line that contains the
-    encoding declaration.
+    encoding declaration.  If the first line matches the second line
+    is ignored.
 
     To aid with platforms such as Windows, which add Unicode BOM marks
     to the beginning of Unicode files, the UTF-8 signature