Completion on current word.
Like M-/ but finds all expansions in the current buffer
and presents suggestions for completion.
With a prefix argument ARG, it searches all buffers accepted by the
function pointed out by `dabbrev-friend-buffer-function' to find the
If the prefix argument is 16 (which comes from C-u C-u),
then it searches *all* buffers. (fn &optional ARG)