doc: add note about bool into coding style

Added a note into the coding style to
highlight the use of a bool within a struct

Signed-off-by: Marko Kovacevic <marko.kovacevic@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>

doc/guides/contributing/coding_style.rst

@@ -247,6 +247,15 @@ Structure Declarations
 * Use of the structures should be by separate variable declarations and those declarations must be extern if they are declared in a header file.
 * Externally visible structure definitions should have the structure name prefixed by ``rte_`` to avoid namespace collisions.
 
+.. note::
+
+    Uses of ``bool`` in structures are not preferred as is wastes space and
+    it's also not clear as to what type size the bool is. A preferred use of
+    ``bool`` is mainly as a return type from functions that return true/false,
+    and maybe local variable functions.
+
+    Ref: `LKML <https://lkml.org/lkml/2017/11/21/384>`_
+
 Queues
 ~~~~~~