[docs] Question mark in grammar notation

[Thomas Oliveira]

Dear Admin,

In http://docs.python.org/py3k/library/string.html#format-string-syntax
a question mark is used at the definition of the grammar for a 
replacement field:

arg_name          ::=  (identifier | integer)?

However, it is not defined in
http://docs.python.org/py3k/reference/introduction.html#notation

Would it be the same as
arg_name          ::=  [identifier | integer]
?

Thank you for your attention,

Thomas Oliveira