bug#10503: 24.0.92; doc string of `echo-keystrokes' is missing important info

[Drew Adams]

The doc string MUST, like the manuals, say what a zero value means.
Otherwise users will wonder why echoing doesn't happen in some context
or how to turn off echoing.
 
This is what the Elisp manual says:
 
"If the value is zero, then command input is not echoed."
 
And this is what the Emacs manual says:
 
"... or zero, meaning don't echo at all."

In GNU Emacs 24.0.92.1 (i386-mingw-nt5.1.2600) of 2012-01-05 on MARVIN
 Windowing system distributor `Microsoft Corp.', version 5.1.2600
 configured using `configure --with-gcc (4.6) --no-opt --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include --ldflags
 -LD:/devel/emacs/libs/gnutls-3.0.9/lib'