Warn that Latex cannot be used in panel tags

See #7958 for background.  This updates the documentation until we have a better solution.

doc/rst/source/subplot.rst

@@ -109,6 +109,8 @@ Optional Arguments (begin mode)
  shift relative to the tag box [default is **2p**\ /**-2p**] and *shade* sets the
  fill style to use for shading [default is **gray50**].
  Append **+v** to increase tag numbers vertically down columns [horizontally across rows].
+ **Note**: Currently you cannot use Latex expressions in the panel tag. The workaround is
+ to place such tags separately via :doc:text.
 
 .. |Add_-B| replace:: |Add_-B_links|
 .. include:: explain_-B.rst_