cmd/go: fix docs for -timeout

The text before CL 45816 was:

	-timeout t
		If a test runs longer than t, panic.
		The default is 10 minutes (10m).

CL 45816 was supposed to be about clarifying test vs test binary,
and it did add the clarification of referring to "duration d",
but it also introduced incorrect text about timeout 0.

The new text in this CL preserves the good change and
eliminates the incorrect one:

	-timeout d
		If a test binary runs longer than duration d, panic.
		The default is 10 minutes (10m).

For #14780.

Change-Id: I4f79d6e48ed9295bc9f34a36aa90d3b03b40d7f5
Reviewed-on: https://go-review.googlesource.com/47571
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -1499,8 +1499,7 @@
 // 	    exhaustive tests.
 //
 // 	-timeout d
-// 	    If the cumulative test time for a package runs longer than
-// 	    duration d, panic. Timeout is disabled if set to 0.
+// 	    If a test binary runs longer than duration d, panic.
 // 	    The default is 10 minutes (10m).
 //
 // 	-v

src/cmd/go/internal/test/test.go

@@ -224,8 +224,7 @@ const testFlag2 = `
 	    exhaustive tests.
 
 	-timeout d
-	    If the cumulative test time for a package runs longer than
-	    duration d, panic. Timeout is disabled if set to 0.
+	    If a test binary runs longer than duration d, panic.
 	    The default is 10 minutes (10m).
 
 	-v