Fix special properties list

* lisp/org.el (org-special-properties): Add missing "CATEGORY".

* doc/org.texi (Special properties): Remove "NOBLOCKING" as a special
  property, since it can only be set through a properties drawer.
  This is a reserved property.

Reported-by: Samuel Loury <konubinix@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/94483>

doc/org.texi

@@ -5368,7 +5368,6 @@ should not be used as keys in the properties drawer:
 @example
 ALLTAGS      @r{All tags, including inherited ones.}
 BLOCKED      @r{"t" if task is currently blocked by children or siblings.}
-NOBLOCKING   @r{"t" if blocking is disabled for this task.}
 CATEGORY     @r{The category of an entry.}
 CLOCKSUM     @r{The sum of CLOCK intervals in the subtree.  @code{org-clock-sum}}
              @r{must be run first to compute the values in the current buffer.}

lisp/org.el

@@ -15363,8 +15363,9 @@ a *different* entry, you cannot use these techniques."
 ;;; Properties API
 
 (defconst org-special-properties
-  '("ALLTAGS" "BLOCKED" "CLOCKSUM" "CLOCKSUM_T" "CLOSED" "DEADLINE" "FILE"
-    "ITEM" "PRIORITY" "SCHEDULED" "TAGS" "TIMESTAMP" "TIMESTAMP_IA" "TODO")
+  '("ALLTAGS" "BLOCKED" "CATEGORY" "CLOCKSUM" "CLOCKSUM_T" "CLOSED" "DEADLINE"
+    "FILE" "ITEM" "PRIORITY" "SCHEDULED" "TAGS" "TIMESTAMP" "TIMESTAMP_IA"
+    "TODO")
   "The special properties valid in Org mode.
 These are properties that are not defined in the property drawer,
 but in some other way.")