misc fixups

Author: zimbatm

docs/_config.yml

@@ -1,7 +1,7 @@
 remote_theme: numtide/primer
 title: devshell
 description: "per project developer environments"
-url: "https://zimbatm.com"
+url: "https://numtide.github.io/devshell"
 author:
   twitter: NumTide
   github: numtide

mkDevShell/options.nix

@@ -164,6 +164,9 @@ in
       default = { };
       description = ''
         Environment variables to add to the environment.
+
+        If the value is null, it will unset the environment variable.
+        Otherwise, the value will be converted to string before being set.
       '';
       example = {
         GO111MODULE = "on";