bug#13343: 24.3.50; give examples using `gv-define-expander' and `gv-letplace'

[Drew Adams]

(elisp) `Adding Generalized Variables' says only this:
 
  For more control over the expansion, see the macro `gv-define-expander'.
  The macro `gv-letplace' can be useful in defining macros that perform
  similarly to `setf'; for example, the `incf' macro of Common Lisp.
  Consult the source file `gv.el' for more details.
 
That's a complete cop-out.  Give at least one example of using each of
`gv-define-expander' and `gv-letplace'.
 
Telling users to refer to gv.el for information about using these macros
is off-base.  Neither the commentary in that file nor the doc strings of
these two macros help.
 
We should not send users off to analyze the _implementation_ of these
macros in order to figure out how to use them to define generalized
variables.  We should describe what these macros do specifically, and
show how to use them.
 
All the current doc says, to describe these macros, is (a)
`gv-define-expander' gives you "more control" and (b) `gv-letplace' "can
be useful".
 
"See the macro", indeed.  See what?  It's implementation?  Emacs users
deserve better.
 
In GNU Emacs 24.3.50.1 (i386-mingw-nt5.1.2600)
 of 2012-12-31 on ODIEONE
Bzr revision: 111388 rudalics@gmx.at-20121231113513-subz2dazg6yjukzh
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.7) --no-opt --enable-checking --cflags
 -IC:/Devel/emacs/build/include --ldflags -LC:/Devel/emacs/build/lib'