cmd/go: document changes to go test -c and -o

This was missing from CL 466397.

For #15513.

Change-Id: I138b7d76842815f4e702d7fe551aa8968097f75b
Reviewed-on: https://go-review.googlesource.com/c/go/+/500955
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Bypass: Russ Cox <rsc@golang.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index ccf5605a63..bb28756133 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -1799,9 +1799,9 @@
 //	    the package list (if present) must appear before this flag.
 //
 //	-c
-//	    Compile the test binary to pkg.test but do not run it
+//	    Compile the test binary to pkg.test in the current directory but do not run it
 //	    (where pkg is the last element of the package's import path).
-//	    The file name can be changed with the -o flag.
+//	    The file name or target directory can be changed with the -o flag.
 //
 //	-exec xprog
 //	    Run the test binary using xprog. The behavior is the same as
@@ -1814,6 +1814,8 @@
 //	-o file
 //	    Compile the test binary to the named file.
 //	    The test still runs (unless -c or -i is specified).
+//	    If file ends in a slash or names an existing directory,
+//	    the test is written to pkg.test in that directory.
 //
 // The test binary also accepts flags that control execution of the test; these
 // flags are also accessible by 'go test'. See 'go help testflag' for details.