ox-latex: Allow horizontal rules in table math mode

* lisp/ox-latex.el (org-latex--math-table): Handle horizontal rules.
* doc/org.texi (Tables in @LaTeX{} export): Update documentation.

doc/org.texi

@@ -12063,11 +12063,10 @@ layout and contents.  Valid @LaTeX{} attributes include:
 @vindex org-latex-default-table-mode
 Nature of table's contents.  It can be set to @code{table}, @code{math},
 @code{inline-math} or @code{verbatim}.  In particular, when in @code{math} or
-@code{inline-math} mode, every cell is exported as-is, horizontal rules are
+@code{inline-math} mode, every cell is exported as-is and the table is
-ignored and the table will be wrapped in a math environment.  Also,
+wrapped within a math environment.  Also, contiguous tables sharing the same
-contiguous tables sharing the same math mode will be wrapped within the same
+math mode are merged within the same environment.  Default mode is determined
-environment.  Default mode is determined in
+in @code{org-latex-default-table-mode}.
-@code{org-latex-default-table-mode}.
 @item :environment
 @vindex org-latex-default-table-environment
 Environment used for the table.  It can be set to any @LaTeX{} table

etc/ORG-NEWS

@@ -23,6 +23,11 @@ into
 
 : (file (lambda () (sexp)))
 
+
+** New features
+
+*** Horizontal rules are no longer ignored in LaTeX table math mode
+
 * Version 9.0
 
 ** Incompatible changes

lisp/ox-latex.el

@@ -3271,8 +3271,7 @@ This function assumes TABLE has `org' as its `:type' property and
 	 (contents
 	  (mapconcat
 	   (lambda (row)
-	     ;; Ignore horizontal rules.
+	     (if (eq (org-element-property :type row) 'rule) "\\hline"
-	     (when (eq (org-element-property :type row) 'standard)
 	       ;; Return each cell unmodified.
 	       (concat
 		(mapconcat