Help: Clarify PDB_NAME fallback behavior (#15518)

It uses the OUTPUT_NAME if set, not always the target name.

Help/prop_tgt/PDB_NAME.rst

@@ -5,7 +5,8 @@ Output name for the MS debug symbol ``.pdb`` file generated by the
 linker for an executable or shared library target.
 
 This property specifies the base name for the debug symbols file.
-If not set, the logical target name is used by default.
+If not set, the :prop_tgt:`OUTPUT_NAME` target property value or
+logical target name is used by default.
 
 .. |COMPILE_PDB_XXX| replace:: :prop_tgt:`COMPILE_PDB_NAME`
 .. include:: PDB_NOTE.txt