Various changes as mentioned by Jonathan Wakely..
* Fix various spelling typos.
* Clarified the effects of insert_above().
* Remove in-class template lists in favor of injected class names to
follow C++14 cleanups.
* Add missing "root() const" and "shoot() const" from class summaries.
They where specified as general requirements, but got missed in the
classes themselves.
* Added "inorder_first() const" to match above as it generally
convenient for writing algorithms.
* Switch to using range-based for loop to increase clarity in effects.
* Remove bad grammar "either of" in cursors/2.