[util.smartptr.owner.equal] Clarify that "they" are "x and y".

Author: burblebee

source/memory.tex

@@ -4948,8 +4948,7 @@
 \pnum
 \begin{note}
 \tcode{x.owner_equal(y)} is \tcode{true}
-%FIXME: Who are "they"?
-if and only if they share ownership or are both empty.
+if and only if \tcode{x} and \tcode{y} share ownership or are both empty.
 \end{note}
 \end{itemdescr}