Clarify the preferred prefix form to be within loops in the style guide

Contributing.md

@@ -139,7 +139,7 @@ private:
 - Try to avoid using raw pointers (pointers allocated with `new`) as much as possible. There are cases where using a raw pointer is unavoidable, and in these situations it is OK to use them. An example of this is functions from a C library that require them. In cases where it is avoidable, the STL usually has a means to solve this (`vector`, `unique_ptr`, etc).
 - Do not use the `auto` keyword everywhere. While it's nice that the type can be determined by the compiler, it cannot be resolved at 'readtime' by the developer as easily. Use auto only in cases where it is obvious what the type being assigned is (note: 'obvious' means not having to open other files or reading the header file). Some situations where it is appropriate to use `auto` is when iterating over a `std::map` container in a foreach loop, or to shorten the length of container iterator variable declarations.
 - Do not use `using namespace [x];` in headers. Try not to use it at all if you can.
-- The preferred form of the increment and decrement operator is prefix (e.g. `++var`).
+- The preferred form of the increment and decrement operator in for-loops is prefix-form (e.g. `++var`).
 
 ### Headers
 - If a header is not necessary in a certain source file, remove them.