rbtree: add some additional comments for rebalancing cases

While overall the code is very nicely commented, it might not be
immediately obvious from the diagrams what is going on.  Add a very
brief summary of each case.  Opposite cases where the node is the left
child are left untouched.

Link: http://lkml.kernel.org/r/20170719014603.19029-4-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/rbtree.c

@@ -132,7 +132,7 @@ __rb_insert(struct rb_node *node, struct rb_root *root,
 		if (parent != tmp) {	/* parent == gparent->rb_left */
 			if (tmp && rb_is_red(tmp)) {
 				/*
-				 * Case 1 - color flips
+				 * Case 1 - node's uncle is red (color flips).
 				 *
 				 *       G            g
 				 *      / \          / \
@@ -155,7 +155,8 @@ __rb_insert(struct rb_node *node, struct rb_root *root,
 			tmp = parent->rb_right;
 			if (node == tmp) {
 				/*
-				 * Case 2 - left rotate at parent
+				 * Case 2 - node's uncle is black and node is
+				 * the parent's right child (left rotate at parent).
 				 *
 				 *      G             G
 				 *     / \           / \
@@ -179,7 +180,8 @@ __rb_insert(struct rb_node *node, struct rb_root *root,
 			}
 
 			/*
-			 * Case 3 - right rotate at gparent
+			 * Case 3 - node's uncle is black and node is
+			 * the parent's left child (right rotate at gparent).
 			 *
 			 *        G           P
 			 *       / \         / \