gdb/dwarf: add doc for cutu_reader::is_dummy

Change-Id: Ifb80557187c12822bdea7ad400c32c3dce968a7f
Approved-By: Tom Tromey <tom@tromey.com>

gdb/dwarf2/read.h

diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index b6ff433..d6d119e 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -945,6 +945,11 @@ class cutu_reader
 
   cutu_reader (cutu_reader &&) = default;
 
+  /* Return true if either:
+
+      - the unit is empty (just a header without any DIE)
+      - the unit is a partial unit and this cutu_reader was built with SKIP
+	PARTIAL true.  */
   bool is_dummy () const { return m_dummy_p; }
 
   dwarf2_cu *cu () const { return m_cu; }