[unique.ptr.single.modifiers] Use 'if and only if' when calling deleter. (#1255)

Fixes #248.

Author: jensmaurer

source/utilities.tex

@@ -8821,7 +8821,7 @@
 well-defined behavior, and shall not throw exceptions.
 
 \pnum
-\effects Assigns \tcode{p} to the stored pointer, and then if the old value of the
+\effects Assigns \tcode{p} to the stored pointer, and then if and only if the old value of the
 stored pointer, \tcode{old_p}, was not equal to \tcode{nullptr}, calls
 \tcode{get_deleter()(old_p)}. \begin{note} The order of these operations is significant
 because the call to \tcode{get_deleter()} may destroy \tcode{*this}. \end{note}