[over.inc] Add 'non-static' to the description of a member operator++

or operator-- to avoid confusion when comparing this wording to that
of similar paragraphs in other clauses. The normative rule already
exists in [over.oper]p6.

Fixes #346.

Author: zygoloid

source/overloading.tex

@@ -3148,12 +3148,12 @@
 implements the prefix and postfix
 \tcode{++}
 operator.
-If this function is a member function with no parameters, or a non-member
+If this function is a non-static member function with no parameters, or a non-member
 function with one parameter,
 it defines the prefix increment operator
 \tcode{++}
 for objects of that type.
-If the function is a member function with one parameter (which shall be of type
+If the function is a non-static member function with one parameter (which shall be of type
 \tcode{int})
 or a non-member function with two parameters (the second of which shall be of type
 \tcode{int}),