Clarify `org-use-sub-superscripts' and related options

* lisp/org.el (org-use-sub-superscripts): Update the docstring
explaining that (...) is understood as sub/superscript, even when
there are spaces inside.  Add a note that the underlying markup is not
altered.
* lisp/ox.el (org-export-with-sub-superscripts): Link to the
`org-use-sub-superscripts' docstring instead of duplicating the
docstring text.
* doc/org-manual.org (Subscripts and Superscripts): Clarify the
differences between `org-export-with-sub-superscripts' and
`org-use-sub-superscripts'.  Add a note that underlying markup does
not change.

Link: https://orgmode.org/list/87plvproor.fsf@localhost

doc/org-manual.org

@@ -11274,15 +11274,20 @@ the radius of Alpha Centauri is R_{Alpha Centauri} = 1.28 x R_{sun}.
 #+end_example
 
 #+vindex: org-use-sub-superscripts
+#+vindex: org-export-with-sub-superscripts
 If you write a text where the underscore is often used in a different
 context, Org's convention to always interpret these as subscripts can
-get in your way.  Configure the variable ~org-use-sub-superscripts~ to
-change this convention.  For example, when setting this variable to
-~{}~, =a_b= is not interpreted as a subscript, but =a_{b}= is.
+get in your way.  Configure the variable ~org-use-sub-superscripts~
+and/or ~org-export-with-sub-superscripts~ to change this convention.
+For example, when setting these variables to ~{}~, =a_b= is not
+displayed/exported[fn::The underlying markup still remains a
+sub/superscript.  Only the visual display and export behavior
+changes.] as a subscript, but =a_{b}= is.
 
-You can set ~org-use-sub-superscripts~ in a file using the export
-option =^:= (see [[*Export Settings][Export Settings]]).  For example, =#+OPTIONS: ^:{}=
-sets ~org-use-sub-superscripts~ to ~{}~ and limits super- and
+You can set both ~org-use-sub-superscripts~
+~org-export-with-sub-superscripts~ in a file using the export option
+=^:= (see [[*Export Settings][Export Settings]]).  For example,
+=#+OPTIONS: ^:{}= sets the two options to ~{}~ and limits super- and
 subscripts to the curly bracket notation.
 
 You can also toggle the visual display of super- and subscripts:

lisp/org.el

@@ -1107,19 +1107,26 @@ will be parsed as single sub- or superscripts:
  10^-12  or   10^-tau    a leading sign with digits or a word
  x^2-y^3                 will be read as x^2 - y^3, because items are
 			 terminated by almost any nonword/nondigit char.
- x_{i^2} or   x^(2-i)    braces or parenthesis do grouping.
+ x^(2 - i)               expression inside round braces, including the
+                         braces is read as a sub/superscript.
+ x_{i^2}                 curly braces do grouping; braces are not
+                         considered a part of the sub/superscript.
 
 Still, ambiguity is possible.  So when in doubt, use {} to enclose
 the sub/superscript.  If you set this variable to the symbol `{}',
-the braces are *required* in order to trigger interpretations as
+the curly braces are *required* in order to trigger interpretations as
 sub/superscript.  This can be helpful in documents that need \"_\"
-frequently in plain text."
+frequently in plain text.
+
+Setting this variable does not change Org mode markup.  Org mode will
+still parse the matching text as sub/superscript internally.  It is
+only the visual appearance that will be changed."
   :group 'org-startup
   :version "24.4"
   :package-version '(Org . "8.0")
   :type '(choice
 	  (const :tag "Always interpret" t)
-	  (const :tag "Only with braces" {})
+	  (const :tag "Only with curly braces" {})
 	  (const :tag "Never interpret" nil)))
 
 (defcustom org-startup-with-beamer-mode nil

lisp/ox.el

@@ -684,31 +684,14 @@ e.g. \"stat:nil\""
   "Non-nil means interpret \"_\" and \"^\" for export.
 
 If you want to control how Org displays those characters, see
-`org-use-sub-superscripts'.  `org-export-with-sub-superscripts'
-used to be an alias for `org-use-sub-superscripts' in Org <8.0,
-it is not anymore.
+`org-use-sub-superscripts'.
 
 When this option is turned on, you can use TeX-like syntax for
 sub- and superscripts and see them exported correctly.
 
 You can also set the option with #+OPTIONS: ^:t
 
-Several characters after \"_\" or \"^\" will be considered as a
-single item - so grouping with {} is normally not needed.  For
-example, the following things will be parsed as single sub- or
-superscripts:
-
- 10^24   or   10^tau     several digits will be considered 1 item.
- 10^-12  or   10^-tau    a leading sign with digits or a word
- x^2-y^3                 will be read as x^2 - y^3, because items are
-			 terminated by almost any nonword/nondigit char.
- x_{i^2} or   x^(2-i)    braces or parenthesis do grouping.
-
-Still, ambiguity is possible.  So when in doubt, use {} to enclose
-the sub/superscript.  If you set this variable to the symbol `{}',
-the braces are *required* in order to trigger interpretations as
-sub/superscript.  This can be helpful in documents that need \"_\"
-frequently in plain text."
+See `org-use-sub-superscripts' docstring for more details."
   :group 'org-export-general
   :version "24.4"
   :package-version '(Org . "8.0")