* add resize(int, NoChange) and resize(NoChange, int)

* add missing assert in resize(int)
* add examples for all resize variants
* expand docs (part of which is from Tim Hutt's e-mail)

Eigen/src/Core/util/Constants.h

@@ -241,6 +241,8 @@ enum {
   DontAlign = 0x2
 };
 
+enum NoChange_t { NoChange };
+
 enum {
   IsDense         = 0,
   IsSparse        = SparseBit,