pkg/tool/exec: Run does not exececute shell commands

It has always been an abstraction over os/exec.Command, that is,
directly calling a program with some arguments and not a shell.
The top-level godoc was misleading users into thinking
they could use shell syntax like pipes or redirections.

While here, make the cmd docs a bit clearer as well.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I8daa362deac7cb5380f39511f60908c9f13627b9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200524
Reviewed-by: Matthew Sackman <matthew@cue.works>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

pkg/tool/exec/exec.cue

@@ -14,13 +14,15 @@
 
 package exec
 
-// Run executes the given shell command.
+// Run executes a program with the given arguments.
 Run: {
 	$id: *"tool/exec.Run" | "exec" // exec for backwards compatibility
 
-	// cmd is the command to run.
+	// cmd is a non-empty list holding the program name to run
+	// and the arguments to be passed to it.
+	//
 	// Simple commands can use a string, which is split by white space characters.
-	// If any arguments include white space, use the list form.
+	// If any arguments include white space, or for clarity, use the list form.
 	cmd: string | [string, ...string]
 
 	// dir specifies the working directory of the command.