Make doc strings and argument names consistent where necessary.

Make doc strings and argument names consistent where necessary.
Moved user variables to own section.
(bookmark-alist): Better doc string.
(bookmark-insert-current-bookmark): Doc string.
(bookmark-insert-buffer-name): Doc string.
(bookmark-set): Take optional NAME argument.
(bookmark-bmenu-select): Use member to test for string's presence.
Use a more efficent test at end, as suggested by Mikio Nakajima.