kernel: clarify thread->stack_info documentation

ARC/ARM are not properly doing this at the moment but this will
be corrected in later patches.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

include/kernel.h

diff --git a/include/kernel.h b/include/kernel.h
index bb061c3..865316c 100644
--- a/include/kernel.h
+++ b/include/kernel.h
@@ -497,9 +497,15 @@
 #if defined(CONFIG_THREAD_STACK_INFO)
 /* Contains the stack information of a thread */
 struct _thread_stack_info {
-	/* Stack Start */
+	/* Stack Start - Identical to K_THREAD_STACK_BUFFER() on the stack
+	 * object. Represents thread-writable stack area without any extras.
+	 */
 	u32_t start;
-	/* Stack Size */
+
+	/* Stack Size - Thread writable stack buffer size. Represents
+	 * the size of the actual area, starting from the start member,
+	 * that should be writable by the thread
+	 */
 	u32_t size;
 };