Deprecate `org-effort-durations'

* lisp/org.el (org-effort-duration): Remove unused variable.
* lisp/org-compat.el (org-effort-durations): Make
  `org-effort-duration' an alias for `org-duration-units'.
* lisp/org-clock.el (org-clock-modify-effort-estimate): Fix docstring.
* doc/org-manual.org (Footnotes): Clarify footnote.

The variable is inactive since introduction of "org-duration" library.

doc/org-manual.org

@@ -21016,9 +21016,9 @@ configured keys.
 [fn:56] If more than one summary type applies to the same property,
 the parent values are computed according to the first of them.
 
-[fn:57] An age is defined as a duration, using effort modifiers
-defined in ~org-effort-durations~, e.g., =3d 1h=.  If any value in the
-column is as such, the summary is also an effort duration.
+[fn:57] An age can be defined as a duration, using units defined in
+~org-duration-units~, e.g., =3d 1h=.  If any value in the column is as
+such, the summary is also expressed as a duration.
 
 [fn:58] Please note that the =COLUMNS= definition must be on a single
 line; it is wrapped here only because of formatting constraints.

lisp/org-clock.el

@@ -733,8 +733,8 @@ previous clocking intervals."
 VALUE can be a number of minutes, or a string with format hh:mm or mm.
 When the string starts with a + or a - sign, the current value of the effort
 property will be changed by that amount.  If the effort value is expressed
-as an `org-effort-durations' (e.g. \"3h\"), the modified value will be
-converted to a hh:mm duration.
+as an unit defined in `org-duration-units' (e.g. \"3h\"), the modified
+value will be converted to a hh:mm duration.
 
 This command will update the \"Effort\" property of the currently
 clocked item, and the value displayed in the mode line."

lisp/org-compat.el

@@ -228,6 +228,9 @@ Counting starts at 1."
 (define-obsolete-function-alias 'org-remove-from-invisibility-spec
   'remove-from-invisibility-spec "Org 9.2")
 
+(define-obsolete-variable-alias 'org-effort-durations 'org-duration-units
+  "Org 9.2")
+
 (defun org-in-fixed-width-region-p ()
   "Non-nil if point in a fixed-width region."
   (save-match-data

lisp/org.el

@@ -17563,29 +17563,6 @@ If there is already a time stamp at the cursor position, update it."
       (org-insert-time-stamp
        (encode-time 0 0 0 (nth 1 cal-date) (car cal-date) (nth 2 cal-date))))))
 
-(defcustom org-effort-durations
-  `(("min" . 1)
-    ("h" . 60)
-    ("d" . ,(* 60 8))
-    ("w" . ,(* 60 8 5))
-    ("m" . ,(* 60 8 5 4))
-    ("y" . ,(* 60 8 5 40)))
-  "Conversion factor to minutes for an effort modifier.
-
-Each entry has the form (MODIFIER . MINUTES).
-
-In an effort string, a number followed by MODIFIER is multiplied
-by the specified number of MINUTES to obtain an effort in
-minutes.
-
-For example, if the value of this variable is ((\"hours\" . 60)), then an
-effort string \"2hours\" is equivalent to 120 minutes."
-  :group 'org-agenda
-  :version "26.1"
-  :package-version '(Org . "8.3")
-  :type '(alist :key-type (string :tag "Modifier")
-		:value-type (number :tag "Minutes")))
-
 (defcustom org-image-actual-width t
   "Should we use the actual width of images when inlining them?