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 <[email protected]>
TryBot-Bypass: Russ Cox <[email protected]>

Author: rsc

src/cmd/go/alldocs.go

@@ -145,9 +145,9 @@ In addition to the build flags, the flags handled by 'go test' itself are:
 	    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
@@ -160,6 +160,8 @@ In addition to the build flags, the flags handled by 'go test' itself are:
 	-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.