[exception] Rephrase to avoid overfull hbox (#1057)

See #693.

Author: jensmaurer

source/support.tex

@@ -2798,7 +2798,7 @@
 
 \pnum
 \postconditions If \tcode{*this} and \tcode{rhs} both have dynamic type \tcode{exception}
-then \tcode{strcmp(what(), rhs.what())} shall equal 0.
+then the value of the expression \tcode{strcmp(what(), rhs.what())} shall equal 0.
 \end{itemdescr}
 
 \indexlibrary{\idxcode{exception}!destructor}%