[m-rev.] for review: fix definition fo foreign_export_enum pragmas

[Julien Fischer]

For review by anyone.

Minor fix to the definition of foreign_export_enum pragmas.

doc/reference_manual.texi:
 	Make it clear that the attributes argument of a foreign_export_enum
 	pragma can only be omitted if the overrides argument is omitted too.

Julien.

diff --git a/doc/reference_manual.texi b/doc/reference_manual.texi
index 7a0bc27..c6c1a16 100644
--- a/doc/reference_manual.texi
+++ b/doc/reference_manual.texi
@@ -7350,7 +7350,7 @@ This argument may be omitted if @var{Overrides} is empty.

  The argument @var{Attributes} is a list of optional attributes.
  If empty, it may be omitted from the @samp{pragma foreign_export_enum}
-declaration.
+declaration provided that the @var{Overrides} argument has also been omitted.
  The following attributes must be supported by all Mercury implementations.

  @table @asis