spec: describe constraint parsing ambiguity and work-around more precisely

The new description matches the implementation (CL 370774).

Also, in the section on type constraints, use "defines" instead of
"determines" because the constraint interface defines the type set
which is precisely the set of acceptable type arguments.

For #49482.

Change-Id: I6f30f49100e8ba8bec0a0f1b450f88cae54312eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/372874
Trust: Robert Griesemer <[email protected]>
Run-TryBot: Robert Griesemer <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: griesemer

doc/go_spec.html

@@ -2604,7 +2604,8 @@ <h3 id="Type_parameter_lists">Type parameter lists</h3>
 <p>
 A parsing ambiguity arises when the type parameter list for a parameterized type
 declares a single type parameter with a type constraint of the form <code>*C</code>
-or <code>(C)</code>:
+or <code>(C)</code> where <code>C</code> is not a (possibly parenthesized)
+<a href="#Types">type literal</a>:
 </p>
 
 <pre>
@@ -2616,17 +2617,19 @@ <h3 id="Type_parameter_lists">Type parameter lists</h3>
 In these rare cases, the type parameter declaration is indistinguishable from
 the expressions <code>P*C</code> or <code>P(C)</code> and the type declaration
 is parsed as an array type declaration.
-To resolve the ambiguity, embed the constraint in an interface:
+To resolve the ambiguity, embed the constraint in an interface or use a trailing
+comma:
 </p>
 
 <pre>
 type T[P interface{*C}] …
+type T[P *C,] …
 </pre>
 
 <h4 id="Type_constraints">Type constraints</h4>
 
 <p>
-A type constraint is an <a href="#Interface_types">interface</a> that determines the
+A type constraint is an <a href="#Interface_types">interface</a> that defines the
 set of permissible type arguments for the respective type parameter and controls the
 operations supported by values of that type parameter.
 </p>