plugin: document support for macOS

All plugins issues I would call bugs now closed, so
(with some amount of optimism) update the plugin documentation.

Change-Id: Ia421c18a166d7cdf599ac86f2336541c1ef42a0d
Reviewed-on: https://go-review.googlesource.com/65670Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/plugin/plugin.go

@@ -13,8 +13,8 @@
 // already part of the program are called. The main function is not run.
 // A plugin is only initialized once, and cannot be closed.
 //
-// The plugin support is currently incomplete, only supports Linux,
-// and has known bugs. Please report any issues.
+// Currently plugins are only supported on Linux and macOS.
+// Please report any issues.
 package plugin
 
 // Plugin is a loaded Go plugin.