Revert "ox-texinfo: Escape @ { } in index"

* lisp/ox-texinfo.el (org-texinfo-keyword): Index keywords are
  specific to Texinfo back-end, so they expect regular Texinfo code.
* doc/org.texi (Indices): Update documentation.

doc/org.texi

@@ -13665,16 +13665,28 @@ entry:
 @subsection Indices
 
 @cindex #+CINDEX
+@cindex concept index, in Texinfo export
+@cindex Texinfo export, index, concept
 @cindex #+FINDEX
+@cindex function index, in Texinfo export
+@cindex Texinfo export, index, function
 @cindex #+KINDEX
+@cindex keystroke index, in Texinfo export
+@cindex Texinfo export, keystroke index
 @cindex #+PINDEX
+@cindex program index, in Texinfo export
+@cindex Texinfo export, program index
 @cindex #+TINDEX
+@cindex data type index, in Texinfo export
+@cindex Texinfo export, data type index
 @cindex #+VINDEX
+@cindex variable index, in Texinfo export
+@cindex Texinfo export, variable index
 The Texinfo export back-end recognizes these indexing keywords if used in the
 Org file: @code{#+CINDEX}, @code{#+FINDEX}, @code{#+KINDEX}, @code{#+PINDEX},
-@code{#+TINDEX}, and @code{#+VINDEX}.  The back-end also recognizes custom
-index definitions that use raw Texinfo code in the Org file (@pxref{Quoting
-Texinfo code}).
+@code{#+TINDEX}, and @code{#+VINDEX}.  Write their value as verbatim Texinfo
+code; in particular, @samp{@{}, @samp{@}} and @samp{@@} characters need to be
+escaped with @samp{@@} if they not belong to a Texinfo command.
 
 @example
 #+CINDEX: Defining indexing entries

lisp/ox-texinfo.el

@@ -970,10 +970,9 @@ contextual information."
 (defun org-texinfo-keyword (keyword _contents info)
   "Transcode a KEYWORD element from Org to Texinfo.
 CONTENTS is nil.  INFO is a plist holding contextual information."
-  (let* ((raw-value (org-element-property :value keyword))
-	 (value (org-texinfo--sanitize-content raw-value)))
+  (let ((value (org-element-property :value keyword)))
     (pcase (org-element-property :key keyword)
-      ("TEXINFO" raw-value)
+      ("TEXINFO" value)
       ("CINDEX" (format "@cindex %s" value))
       ("FINDEX" (format "@findex %s" value))
       ("KINDEX" (format "@kindex %s" value))