Document that diag() returns false,

lib/Test2/Tools/Basic.pm

@@ -41,6 +41,7 @@ sub diag {
     my $ctx = context();
     $ctx->diag( join '', grep { defined $_ } @_ );
     $ctx->release;
+    return 0;
 }
 
 sub note {
@@ -248,6 +249,8 @@ Fire off a failing test (a single Ok event). The name and diagnostics are option
 Write diagnostics messages. All items in C<@messages> will be joined into a
 single string with no separator. When using TAP, diagnostics are sent to STDERR.
 
+Returns false, so as to preserve failure.
+
 =item note(@messages)
 
 Write note-diagnostics messages. All items in C<@messages> will be joined into