Warn when linking a plugin into a non-plugin crate

Fixes #22202.

src/doc/trpl/plugins.md

@@ -39,6 +39,16 @@ If present, arguments passed as `#![plugin(foo(... args ...))]` are not
 interpreted by rustc itself.  They are provided to the plugin through the
 `Registry`'s [`args` method](../rustc/plugin/registry/struct.Registry.html#method.args).
 
+In the vast majority of cases, a plugin should *only* be used through
+`#![plugin]` and not through an `extern crate` item.  Linking a plugin would
+pull in all of libsyntax and librustc as dependencies of your crate.  This is
+generally unwanted unless you are building another plugin.  The
+`plugin_as_library` lint checks these guidelines.
+
+The usual practice is to put compiler plugins in their own crate, separate from
+any `macro_rules!` macros or ordinary Rust code meant to be used by consumers
+of a library.
+
 # Syntax extensions
 
 Plugins can extend Rust's syntax in various ways. One kind of syntax extension