Address review comments

Author: cpplearner

source/iostreams.tex

@@ -4588,8 +4588,8 @@
 
 \pnum
 The currently imbued locale in \tcode{is} is used
-to determine whether or not the next input character is whitespace.
-The character \tcode{c} is a whitespace character if
+to determine whether the next input character is whitespace:
+The character \tcode{c} is a whitespace character if and only if
 \tcode{ct.is(ct.space, c)} is \tcode{true},
 where \tcode{ct} is \tcode{use_facet<ctype<charT>>(is.getloc())}.