Introduce S-actionable and S-unactionable labels

I've noticed that a significant fraction of issues are inert.
They are valid, acknowledged and useful, but effectively can't be
fixed for variety of reasons (no reproduction, dependencies on some
other issues, no review capacity, etc).

Marking issues that can be fixed by just applying some elbow grease
seems useful!

Author: matklad

docs/dev/README.md

@@ -35,7 +35,12 @@ https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0
 * [E-easy](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-easy),
   [E-medium](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-medium),
   [E-hard](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-hard),
-  labels are *estimates* for how hard would be to write a fix.
+  [E-unknown](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-unknown),
+  labels are *estimates* for how hard would be to write a fix. Each triaged issue should have one of these labels.
+* [S-actionable](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AS-actionable) and
+  [S-unactionable](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AS-unactionable)
+  specify if there are concrete steps to resolve or advance an issue. Roughly, actionable issues need only work to be fixed,
+  while unactionable ones are effectively wont-fix. Each triaged issue should have one of these labels.
 * [fun](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3Afun)
   is for cool, but probably hard stuff.