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

[Drew Adams]

 Define COMMAND as a prefix command.  COMMAND should be a symbol.
 A new sparse keymap is stored as COMMAND's function definition and its value.
 If a second optional argument MAPVAR is given, the map is stored as
 its value instead of as COMMAND's value; but COMMAND is still defined
 as a function.
 The third optional argument NAME, if given, supplies a menu name
 string for the map.  This is required to use the keymap as a menu.
 This function returns COMMAND.

1. It is not at all clear what the occurrence of "its" in the 4th
   sentence ("If a second...") refers to.  And this part of the doc
   string should explicitly say what MAPVAR is: a symbol that is used as
   a variable - it is given the map as value.

2. The second line of the doc string is too long - should not be more
   than 70 chars long.


In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-06-28 on ODIEONE
Bzr revision: 117431 rgm@gnu.org-20140628015517-eku6hj8mpgcvfnso
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''