tree-walk: move doc to tree-walk.h

tree-walk: move doc to tree-walk.h

Move the documentation from Documentation/technical/api-tree-walking.txt
to tree-walk.h as it's easier for the developers to find the usage
information beside the code instead of looking for it in another doc file.

Documentation/technical/api-tree-walking.txt is removed because the
information it has is now redundant and it'll be hard to keep it up to
date and synchronized with the documentation in the header file.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>