org-capture: Support all capture template target-file variants in Customize

* lisp/org-capture.el (`org-capture-templates'): Adjust the
  `org-capture-templates' defcustom template to support specifying the capture
  target file using either a literal pathname, a function, a variable or a
  form, as documented.  Previously the Customize UI supported specifying only a
  literal pathname.
  (org-capture-templates): Clarify the documentation for the `function' method
  for setting up the capture target.

* doc/org.texi (Template elements): Clarify the `function' method for setting up
  the capture target.

doc/org.texi

@@ -7192,8 +7192,8 @@ A function to find the right location in the file.
 File to the entry that is currently being clocked.
 
 @item (function function-finding-location)
-Most general way, write your own function to find both
-file and location.
+Most general way: write your own function which both visits
+the file and moves point to the right location.
 @end table
 
 @item template

lisp/org-capture.el

@@ -157,8 +157,8 @@ target       Specification of where the captured item should be placed.
                 File to the entry that is currently being clocked
 
              (function function-finding-location)
-                Most general way, write your own function to find both
-                file and location
+                Most general way: write your own function which both visits
+                the file and moves point to the right location
 
 template     The template for creating the capture item.  If you leave this
              empty, an appropriate default template will be used.  See below
@@ -282,7 +282,12 @@ calendar                |  %:type %:date"
   :group 'org-capture
   :version "24.1"
   :type
-  '(repeat
+  (let ((file-variants '(choice :tag "Filename       "
+				(file :tag "Literal")
+				(function :tag "Function")
+				(variable :tag "Variable")
+				(sexp :tag "Form"))))
+  `(repeat
     (choice :value ("" "" entry (file "~/org/notes.org") "")
 	    (list :tag "Multikey description"
 		  (string :tag "Keys       ")
@@ -299,39 +304,39 @@ calendar                |  %:type %:date"
 		  (choice :tag "Target location"
 			  (list :tag "File"
 				(const :format "" file)
-				(file :tag "  File"))
+				,file-variants)
 			  (list :tag "ID"
 				(const :format "" id)
 				(string :tag "  ID"))
 			  (list :tag "File & Headline"
 				(const :format "" file+headline)
-				(file   :tag "  File    ")
+				,file-variants
 				(string :tag "  Headline"))
 			  (list :tag "File & Outline path"
 				(const :format "" file+olp)
-				(file   :tag "  File    ")
+				,file-variants
 				(repeat :tag "Outline path" :inline t
 					(string :tag "Headline")))
 			  (list :tag "File & Regexp"
 				(const :format "" file+regexp)
-				(file   :tag "  File  ")
+				,file-variants
 				(regexp :tag "  Regexp"))
 			  (list :tag "File & Date tree"
 				(const :format "" file+datetree)
-				(file :tag "  File"))
+				,file-variants)
 			  (list :tag "File & Date tree, prompt for date"
 				(const :format "" file+datetree+prompt)
-				(file :tag "  File"))
+				,file-variants)
 			  (list :tag "File & function"
 				(const :format "" file+function)
-				(file :tag "  File    ")
+				,file-variants
 				(sexp :tag "  Function"))
 			  (list :tag "Current clocking task"
 				(const :format "" clock))
 			  (list :tag "Function"
 				(const :format "" function)
 				(sexp :tag "  Function")))
-		  (choice :tag "Template"
+		  (choice :tag "Template       "
 			  (string)
 			  (list :tag "File"
 				(const :format "" file)
@@ -352,7 +357,7 @@ calendar                |  %:type %:date"
 				   ((const :format "%v " :clock-resume) (const t))
 				   ((const :format "%v " :unnarrowed) (const t))
 				   ((const :format "%v " :table-line-pos) (const t))
-				   ((const :format "%v " :kill-buffer) (const t))))))))
+				   ((const :format "%v " :kill-buffer) (const t)))))))))
 
 (defcustom org-capture-before-finalize-hook nil
   "Hook that is run right before a capture process is finalized.