Documentation: Update augmented rbtree documentation

Augmented rbtree is an rbtree with "some" additional data stored in each node.

This data can be used to augment some new functionality to rbtree.

Augmented rbtree is an optional feature built on top of basic rbtree

infrastructure. rbtree user who wants this feature will have an augment

callback function in rb_root initialized.

This callback function will be called from rbtree core routines whenever

a node has a change in one or both of its children. It is the responsibility

of the callback function to recalculate the additional data that is in the

rb node using new children information. Note that if this new additional

data affects the parent node's additional data, then callback function has

to handle it and do the recursive updates.

infrastructure. An rbtree user who wants this feature will have to call the

augmentation functions with the user provided augmentation callback

when inserting and erasing nodes.

On insertion, the user must call rb_augment_insert() once the new node is in

place. This will cause the augmentation function callback to be called for

each node between the new node and the root which has been affected by the

insertion.

When erasing a node, the user must call rb_augment_erase_begin() first to

retrieve the deepest node on the rebalance path. Then, after erasing the

original node, the user must call rb_augment_erase_end() with the deepest

node found earlier. This will cause the augmentation function to be called

for each affected node between the deepest node and the root.

Interval tree is an example of augmented rb tree. Reference -