Doc-fix: mention FP_ZERO as a possible return of qFpClassify()

This follows up on commit 84aea6c091, in
which I forgot to mention this possible return value.

Change-Id: I109bed66bc0fd63d7ee289bfaea65b3d05c6560c
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qnumeric.cpp

@@ -110,6 +110,7 @@ Q_CORE_EXPORT double qInf() { return qt_inf(); }
     \list
     \li FP_NAN not a number
     \li FP_INFINITE infinities (positive or negative)
+    \li FP_ZERO zero (positive or negative)
     \li FP_NORMAL finite with a full mantissa
     \li FP_SUBNORMAL finite with a reduced mantissa
     \endlist