Fix text in TRPL macros guide to match the example

Also, update description of macro invocation syntax:
after rust-lang#20563 there is
a number of additional limitations on macro syntax.

Author: alkor

src/doc/trpl/macros.md

@@ -42,7 +42,7 @@ the pattern in the above code:
 # let input_1 = T::SpecialA(0);
 # let input_2 = T::SpecialA(0);
 macro_rules! early_return {
-    ($inp:expr, $sp:path) => ( // invoke it like `(input_5 SpecialE)`
+    ($inp:expr, $sp:path) => ( // invoke it like `(input_5, SpecialE)`
         match $inp {
             $sp(x) => { return x; }
             _ => {}
@@ -59,7 +59,7 @@ early_return!(input_2, T::SpecialB);
 ~~~~
 
 Macros are defined in pattern-matching style: in the above example, the text
-`($inp:expr $sp:ident)` that appears on the left-hand side of the `=>` is the
+`($inp:expr, $sp:path)` that appears on the left-hand side of the `=>` is the
 *macro invocation syntax*, a pattern denoting how to write a call to the
 macro. The text on the right-hand side of the `=>`, beginning with `match
 $inp`, is the *macro transcription syntax*: what the macro expands to.
@@ -74,6 +74,8 @@ conforms to the following rules:
 2. `$` has special meaning (described below).
 3. The `()`s, `[]`s, and `{}`s it contains must balance. For example, `([)` is
 forbidden.
+4. Some arguments can be followed only by a limited set of separators, to
+avoid ambiguity (described below).
 
 Otherwise, the invocation syntax is free-form.
 
@@ -86,7 +88,8 @@ To take a fragment of Rust code as an argument, write `$` followed by a name
   `foo`.)
 * `expr` (an expression. Examples: `2 + 2`; `if true then { 1 } else { 2 }`;
   `f(42)`.)
-* `ty` (a type. Examples: `int`, `Vec<(char, String)>`, `&T`.)
+* `ty` (a type. Examples: `i32`, `Vec<(char, String)>`, `&T`.)
+* `path` (a path to struct or enum variant. Example: `T::SpecialA`)
 * `pat` (a pattern, usually appearing in a `match` or on the left-hand side of
   a declaration. Examples: `Some(t)`; `(17, 'a')`; `_`.)
 * `block` (a sequence of actions. Example: `{ log(error, "hi"); return 12; }`)
@@ -97,6 +100,12 @@ rules of tokenization apply,
 So `($x:ident -> (($e:expr)))`, though excessively fancy, would designate a macro
 that could be invoked like: `my_macro!(i->(( 2+2 )))`.
 
+To avoid ambiguity, macro invocation syntax must conform to the following rules:
+* `expr` must be followed by `=>`, `,` or `;`.
+* `ty` and `path` must be followed by `=>`, `,`, `:`, `=`, `>` or `as`.
+* `pat` must be followed by `=>`, `,` or `=`.
+* `ident` and `block` can be followed by any token.
+
 ## Invocation location
 
 A macro invocation may take the place of (and therefore expand to) an