Align subbuf-rw sub declarations.

See issue #1974

Author: fluca1978

doc/Type/Buf.pod6

@@ -45,9 +45,14 @@ happens from the very beginning of the buffer:
 
 =head2 routine subbuf-rw
 
-    sub subbuf-rw(Buf:D $buf, $from, $length?)
+Declared as
 
-Returns a writable reference to a part of a buffer. Similar to the C<subbuf-rw> method:
+    multi sub subbuf-rw(Buf:D \b) is rw
+    multi sub subbuf-rw(Buf:D \b, Int() $from) is rw
+    multi sub subbuf-rw(Buf:D \b, $from, $elems) is rw
+
+Returns a writable reference to a part of a buffer.
+Invokes the C<subbuf-rw> method on the specified C<Buf>:
 
     my Buf $b .= new(1,2,3);
     subbuf-rw($b,2,1) = Buf.new(42);