Added specification for order of escape sequence processing.

pep-0498.txt

@@ -208,6 +208,25 @@ Expressions cannot contain ':' or '!' outside of strings or parens,
 brackets, or braces. The exception is that the '!=' operator is
 special cased.
 
+Escape sequences
+----------------
+
+Scanning an f-string for expressions happens after escape sequences
+are decode. Because hex(ord('{')) == 0x7b, that means that the
+f-string f'\u007b4*10}' is decoded to f'{4*10}', which evaluates as
+the integer 40::
+
+  >>> f'\u007b4*10}'
+  '40'
+  >>> f'\x7b4*10}'
+  '40'
+  >>> f'\x7b4*10\N{RIGHT CURLY BRACKET}'
+  '40'
+
+These examples aren't generally useful, they're just to show that
+escape sequences are processed before f-strings are parsed for
+expressions.
+
 Code equivalence
 ----------------