Proc::Async: say, write and print return promises

lib/Type/Proc/Async.pod

@@ -37,7 +37,7 @@ An example that opens an external program for writing:
     use v6;
     my $prog = Proc::Async.new(:w, 'hexdump', '-C');
     my $promise = $prog.start;
-    $prog.write(Buf.new(12, 42));
+    await $prog.write(Buf.new(12, 42));
     $prog.close-stdin;
     await $promise;
 
@@ -128,6 +128,9 @@ C<new> method.
 Write the binary data in C<$b> to the standard input stream of the external
 program.
 
+Returns a L<Promise|/type/Promise> that will be kept once the data has fully
+landed in the input buffer of the external program.
+
 The C<Proc::Async> object must be created for writing (with
 C<Proc::Async.new(:w, $path, @args)>). Otherwise an
 L<X::Proc::Async::OpenForWriting> exception will the thrown.
@@ -142,6 +145,9 @@ L<X::Proc::Async::MustBeStarted> exception is thrown.
 Write the text data in C<$str> to the standard input stream of the external
 program, encoding it as UTF-8.
 
+Returns a L<Promise|/type/Promise> that will be kept once the data has fully
+landed in the input buffer of the external program.
+
 The C<Proc::Async> object must be created for writing (with
 C<Proc::Async.new(:w, $path, @args)>). Otherwise an
 L<X::Proc::Async::OpenForWriting> exception will the thrown.
@@ -156,6 +162,9 @@ L<X::Proc::Async::MustBeStarted> exception is thrown.
 Write the text data in C<$str> and a newline to the standard input stream of the external
 program, encoding it as UTF-8.
 
+Returns a L<Promise|/type/Promise> that will be kept once the data has fully
+landed in the input buffer of the external program.
+
 The C<Proc::Async> object must be created for writing (with
 C<Proc::Async.new(:w, $path, @args)>). Otherwise an
 L<X::Proc::Async::OpenForWriting> exception will the thrown.