Overwrite grammar sections with what was removed from the reference

Between ffc5f1c, when grammar.md was created by copying parts of the
reference, and 8cf2552, when all EBNF was removed from reference.md,
there were parts of the grammar that were updated in reference.md but
not grammar.md, and then they weren't copied over because they existed
already, but they were slightly out of date.

Example: the `path_item : ident | "self" ;` rule in Use declarations was
changed from "mod" to "self" in the reference in 195fd9a but wasn't
updated in the grammar.

Author: carols10cents

src/doc/grammar.md

@@ -253,7 +253,7 @@ The two values of the boolean type are written `true` and `false`.
 ### Symbols
 
 ```antlr
-symbol : "::" "->"
+symbol : "::" | "->"
        | '#' | '[' | ']' | '(' | ')' | '{' | '}'
        | ',' | ';' ;
 ```
@@ -342,7 +342,7 @@ path_glob : ident [ "::" [ path_glob
                           | '*' ] ] ?
           | '{' path_item [ ',' path_item ] * '}' ;
 
-path_item : ident | "mod" ;
+path_item : ident | "self" ;
 ```
 
 ### Functions
@@ -424,7 +424,7 @@ See [Use declarations](#use-declarations).
 ## Attributes
 
 ```antlr
-attribute : "#!" ? '[' meta_item ']' ;
+attribute : '#' '!' ? '[' meta_item ']' ;
 meta_item : ident [ '=' literal
                   | '(' meta_seq ')' ] ? ;
 meta_seq : meta_item [ ',' meta_seq ] ? ;
@@ -515,8 +515,7 @@ struct_expr : expr_path '{' ident ':' expr
 ### Block expressions
 
 ```antlr
-block_expr : '{' [ view_item ] *
-                 [ stmt ';' | item ] *
+block_expr : '{' [ stmt ';' | item ] *
                  [ expr ] '}' ;
 ```
 
@@ -537,7 +536,7 @@ field_expr : expr '.' ident ;
 ```antlr
 array_expr : '[' "mut" ? array_elems? ']' ;
 
-array_elems : [expr [',' expr]*] | [expr ',' ".." expr] ;
+array_elems : [expr [',' expr]*] | [expr ';' expr] ;
 ```
 
 ### Index expressions