Add documentation on how to debug memory pool sizes

(cherry picked from commit a215eba50eae2320bf9e10499f6b05d9c7639074)

doc/doxygen/main_page.h

@@ -204,6 +204,15 @@
  * Read especially sections "Cache coherency" and "Buffer alignment".
  */
 
+/**
+ * @page mem_err Debugging memory pool sizes
+ * If you have issues with lwIP and you are using memory pools, check that your pools
+ * are correctly sized.\n
+ * To debug pool sizes, \#define LWIP_STATS and MEMP_STATS to 1. Check the global variable
+ * lwip_stats.memp[] using a debugger. If the "err" member of a pool is > 0, the pool
+ * may be too small for your application and you need to increase its size.
+ */
+
 /**
  * @page bugs Reporting bugs
  * Please report bugs in the lwIP bug tracker at savannah.\n