Fill in grammar for Statements

Some of this text is duplicated in the reference (and belongs there) so
remove it. It says item grammar is the same, so point to that grammar
section.

src/doc/grammar.md

@@ -434,26 +434,19 @@ meta_seq : meta_item [ ',' meta_seq ] ? ;
 
 ## Statements
 
-**FIXME:** grammar?
+```antlr
+stmt : decl_stmt | expr_stmt ;
+```
 
 ### Declaration statements
 
-**FIXME:** grammar?
-
-A _declaration statement_ is one that introduces one or more *names* into the
-enclosing statement block. The declared names may denote new variables or new
-items.
+```antlr
+decl_stmt : item | let_decl ;
+```
 
 #### Item declarations
 
-**FIXME:** grammar?
-
-An _item declaration statement_ has a syntactic form identical to an
-[item](#items) declaration within a module. Declaring an item — a
-function, enumeration, structure, type, static, trait, implementation or module
-— locally within a statement block is simply a way of restricting its
-scope to a narrow region containing all of its uses; it is otherwise identical
-in meaning to declaring the item outside the statement block.
+See [Items](#items).
 
 #### Variable declarations
 
@@ -464,7 +457,9 @@ init : [ '=' ] expr ;
 
 ### Expression statements
 
-**FIXME:** grammar?
+```antlr
+expr_stmt : expr ';' ;
+```
 
 ## Expressions