Document special use of target_type field in Ada structs.

This is a comment that I wrote not too long ago, I believe, as part
of a submission process, and somehow did not get to commit.  It explains
how the TYPE_TARGET_TYPE is used by Ada in the case of dynamic records/
unions.

2010-04-24  Joel Brobecker  <brobecker@adacore.com>

        * gdbtypes.h (struct main_type): Expand comment about target_type
        field.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2010-04-27  Joel Brobecker  <brobecker@adacore.com>
+
+	* gdbtypes.h (struct main_type): Expand comment about target_type
+	field.
+
 2010-04-27  Joel Brobecker  <brobecker@adacore.com>
 
 	* symfile.c (reread_symbols): Also search for file in libraries.

gdb/gdbtypes.h

@@ -459,6 +459,9 @@ struct main_type
      For a function or method type, describes the type of the return value.
      For a range type, describes the type of the full range.
      For a complex type, describes the type of each coordinate.
+     For a special record or union type encoding a dynamic-sized type
+     in GNAT, a memoized pointer to a corresponding static version of
+     the type.
      Unused otherwise.  */
 
   struct type *target_type;