[concept.destructible] Fix meaningless phrase "potentially-throwing,

even if non-throwing".

By definition, non-throwing is the opposite of potentially-throwing for
an exception-specification ([except.spec]p1).

Author: zygoloid

source/concepts.tex

@@ -686,7 +686,8 @@
 \pnum
 \begin{note}
 Unlike the \oldconcept{Destructible} requirements~(\tref{destructible}), this
-concept forbids destructors that are potentially throwing, even if non-throwing.
+concept forbids destructors that are potentially throwing, even if a particular
+invocation of the destructor does not actually throw.
 \end{note}
 \end{itemdescr}