Either Brandl or Peterson can and typically will change the .rst source
if given the exact new text.  For me to write that, I need to know the
grammar you actually implemented.  Did you, in essence, simply change

field_name        ::=  (identifier | integer) ("." attribute_name | "["
element_index "]")*

to (in essence)

field_name        ::=  (identifier | integer | ) ("." attribute_name |
"[" element_index "]")*

with the proviso that integers and blanks not be mixed in the same
string, so that{.attr} and {[dex]} become legal?  Or are those still
illegal because only totally blank field names are allowed, so that the
new field_name rule is essentially

field_name        ::=  ((identifier | integer) ("." attribute_name | "["
element_index "]")*) | ( )

(with the same proviso).

The existing doc text after the grammar box is slightly ambiguous or
contradictory in that it first says that field names *are* ints or names
and then says, correctly, that they *begin* with an int or name. (I
would like to fix this in addition to adding a sentence.) Hence 'blank
field name' can have two slightly different meanings and hence the
question above.