Doc fixes in avl-tree.el

Doc fixes in avl-tree.el

* lisp/emacs-lisp/avl-tree.el (avl-tree--root)
(avl-tree--dir-to-sign, avl-tree--sign-to-dir)
(avl-tree--del-balance, avl-tree--enter-balance)
(avl-tree--do-copy, avl-tree--stack-repopulate, avl-tree-empty)
(avl-tree-delete, avl-tree-member, avl-tree-member-p)
(avl-tree-map, avl-tree-mapc, avl-tree-mapf, avl-tree-mapcar)
(avl-tree-copy, avl-tree-clear, avl-tree-stack)
(avl-tree-stack-first): Fix doc strings to be less verbose and to
have the first line a complete sentence.