Move explanation that references lexical grammar to the grammar

Reword so that the references to things above/below are up to date and a
bit more specific.

src/doc/grammar.md

@@ -179,6 +179,12 @@ lit_suffix : ident;
 literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit | bool_lit ] lit_suffix ?;
 ```
 
+The optional `lit_suffix` production is only used for certain numeric literals,
+but is reserved for future extension. That is, the above gives the lexical
+grammar, but a Rust parser will reject everything but the 12 special cases
+mentioned in [Number literals](reference.html#number-literals) in the
+reference.
+
 #### Character and string literals
 
 ```antlr

src/doc/reference.md

@@ -130,11 +130,6 @@ of tokens, that immediately and directly denotes the value it evaluates to,
 rather than referring to it by name or some other evaluation rule. A literal is
 a form of constant expression, so is evaluated (primarily) at compile time.
 
-The optional suffix is only used for certain numeric literals, but is
-reserved for future extension, that is, the above gives the lexical
-grammar, but a Rust parser will reject everything but the 12 special
-cases mentioned in [Number literals](#number-literals) below.
-
 #### Examples
 
 ##### Characters and strings