[Concurrency] Improve comments based on review feedback.

Author: DougGregor

include/swift/AST/Decl.h

@@ -5913,7 +5913,8 @@ class AbstractFunctionDecl : public GenericContext, public ValueDecl {
   /// Retrieve the location of the 'throws' keyword, if present.
   SourceLoc getThrowsLoc() const { return ThrowsLoc; }
 
-  /// Returns true if the function is async.
+  /// Returns true if the function is marked as `async`. The
+  /// type of the function will be `async` as well.
   bool hasAsync() const { return Bits.AbstractFunctionDecl.Async; }
 
   /// Returns true if the function body throws.

lib/Parse/ParseDecl.cpp

@@ -7425,6 +7425,7 @@ Parser::parseDeclInit(ParseDeclOptions Flags, DeclAttributes &Attributes) {
   }
 
   // Initializers cannot be 'async'.
+  // FIXME: We should be able to lift this restriction.
   if (asyncLoc.isValid()) {
     diagnose(asyncLoc, diag::async_init)
       .fixItRemove(asyncLoc);

lib/Sema/TypeCheckProtocol.h

@@ -234,7 +234,7 @@ enum class MatchKind : uint8_t {
   /// The witness would match if an additional requirement were met.
   MissingRequirement,
 
-  /// The witness is async, but the requirement does not.
+  /// The witness and requirement disagree on 'async'.
   AsyncConflict,
 
   /// The witness throws, but the requirement does not.