Attribute grammar in reference manual allowed `#[foo, bar]`, which does not match parser behavior.
Also rename nonterminals to match parser code.
Fix #13825.
## Attributes
~~~~ {.notrust .ebnf .gram}
-attribute : '#' '!' ? '[' attr_list ']' ;
-attr_list : attr [ ',' attr_list ]* ;
-attr : ident [ '=' literal
- | '(' attr_list ')' ] ? ;
+attribute : '#' '!' ? '[' meta_item ']' ;
+meta_item : ident [ '=' literal
+ | '(' meta_seq ')' ] ? ;
+meta_seq : meta_item [ ',' meta_seq ]* ;
~~~~
Static entities in Rust — crates, modules and items — may have _attributes_