bug#44362: Grep pipe doc needs to warn about --null

[積丹尼 Dan Jacobson]

(info "(emacs) Grep Searching") says

        Your command need not simply run ‘grep’; you can use any shell
     command that produces output in the same format.  For instance, you can
     chain ‘grep’ commands, like this:

          grep -nH -e foo *.el | grep bar | grep toto

OK, but it also needs to mention:

To have this currently work, be sure to remove the "--null" currently
added to today's prompt,

    Run grep (like this): grep --color -nH --null -e

Else you will end up with

    Binary file (standard input) matches

emacs-version "27.1"