Default to clojure.core ns for Grimoire

This is necessary for special forms, which do not have ns. Grimoire defaults to
`clojure.core`.

Author: xiongtx

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### New Features
 
+* [#2012](https://github.com/clojure-emacs/cider/pull/2007): Support special forms in `cider-apropos` and `cider-grimoire-lookup`.
 * [#2007](https://github.com/clojure-emacs/cider/pull/2007): Fontify code blocks from `cider-grimoire` if possible.
 * [#1990](https://github.com/clojure-emacs/cider/issues/1990): Add new customation variable `cider-save-files-on-cider-refresh` to allow auto-saving buffers when `cider-refresh` is called.
 * Add new function `cider-load-all-files`, along with menu bar update.

cider-grimoire.el

@@ -91,10 +91,13 @@ opposite of what that option dictates."
     (current-buffer)))
 
 (defun cider-grimoire-lookup (symbol)
-  "Look up the grimoire documentation for SYMBOL."
+  "Look up the grimoire documentation for SYMBOL.
+
+If SYMBOL is a special form, the clojure.core ns is used, as is
+Grimoire's convention."
   (if-let ((var-info (cider-var-info symbol)))
       (let ((name (nrepl-dict-get var-info "name"))
-            (ns (nrepl-dict-get var-info "ns"))
+            (ns (nrepl-dict-get var-info "ns" "clojure.core"))
             (url-request-method "GET")
             (url-request-extra-headers `(("Content-Type" . "text/plain"))))
         (url-retrieve (cider-grimoire-url name ns)