os/exec: document lack of implicit shell usage or globbing

Fixes #20894

Change-Id: I0c0e906964bbd789317d07f245e64e3d61ecfa8c
Reviewed-on: https://go-review.googlesource.com/47550Reviewed-by: Russ Cox <rsc@golang.org>

src/os/exec/exec.go

@@ -6,6 +6,15 @@
 // easier to remap stdin and stdout, connect I/O with pipes, and do other
 // adjustments.
 //
+// Unlike the "system" library call from C and other languages, the
+// os/exec package intentionally does not invoke the system shell and
+// does not expand any glob patterns or handle other expansions,
+// pipelines, or redirections typically done by shells. The package
+// behaves more like C's "exec" family of functions. To expand glob
+// patterns, either call the shell directly, taking care to escape any
+// dangerous input, or use the path/filepath package's Glob function.
+// To expand environment variables, use package os's ExpandEnv.
+//
 // Note that the examples in this package assume a Unix system.
 // They may not run on Windows, and they do not run in the Go Playground
 // used by golang.org and godoc.org.