bug#34288: Mention how to trigger ':' without filemane

[積丹尼 Dan Jacobson]

Regarding (info "(grep) Context Line Control")

To get exactly this great output
$ seq 33|grep --context=4 --label= --with-filename 5$
without further tips on the page, the user won't figure it out.

Even worse, if the input is from a file, not stdin, even I can't figure
it out.

(P.S., to be sure we are both looking at the same thing
$ seq 33|grep --context=4 --label= --with-filename 5$|cksum
1444160178 105
)