bug#4728: 23.1; doc of `buffer-name'

[Drew Adams]

Doc string:
 
 Return the name of BUFFER, as a string.
 With no argument or nil as argument, return the name of the current
 buffer.
 
Elisp manual, node Buffer Names:
 
 This function returns the name of BUFFER as a string.  BUFFER
 defaults to the current buffer.
 
 If `buffer-name' returns `nil', it means that BUFFER has been
 killed.  *Note Killing Buffers::.
 
The doc string suggests that the function always returns a string,
which the manual belies.  The doc string needs to mention the possible
nil return value.
 
 
 
In GNU Emacs 23.1.1 (i386-mingw-nt5.1.2600)
 of 2009-07-29 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4)'