bug#18092: 24.4.50; doc string of `define-prefix-command'

[Drew Adams <drew.adams@oracle.com>]

Things are actually worse than that.

The notion of "prefix command" is not introduced or explained anywhere.
It is not in either the Emacs manual or the Elisp manual. 

`define-prefix-command' has been around for a long time, but it is a
poorly chosen name.  The symbol is NOT defined as a command or even
as a function (it is fboundp but not functionp or commandp).
This name is misleading and confusing.

The Elisp manual speaks better of `define-prefix-command', by saying
that it "prepares SYMBOL for use as a prefix key's binding" - nothing
about "prefix command".  Such wording should be used also for the doc
string.

It would even be good to rename the function (keeping
`define-prefix-command' as a deprecated alias).  Maybe something like
`prepare-symbol-for-prefix-key'.

The doc should also make clear that the value it puts in the function
cell is a new, empty keymap.  Thus, if you do
(define-prefix-command 'foo), and then you define keys in that keymap,
and then you do (define-prefix-command 'foo) again, the keys you
bound earlier are lost (since a new map is now used for foo).