Update get_args documentation

This patch adds argument compilation documentation, expanding on the
already existing comments, giving a more thorough explanation of
the source of the arguments used in the final argument string.

gdb/ChangeLog:

	* compile/compile.c (get_args): Add additional comments
	explaining function.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2018-02-27  Phil Muldoon  <pmuldoon@redhat.com>
+
+	* compile/compile.c (get_args): Add additional comments
+	explaining function.
+
 2018-02-27  Simon Marchi  <simon.marchi@polymtl.ca>
 	    Tom Tromey  <tom@tromey.com>
 

gdb/compile/compile.c

@@ -393,9 +393,25 @@ filter_args (int *argcp, char **argv)
   *destv = NULL;
 }
 
-/* Produce final vector of GCC compilation options.  First element is target
+/* Produce final vector of GCC compilation options.
-   size ("-m64", "-m32" etc.), optionally followed by DW_AT_producer options
+
-   and then compile-args string GDB variable.  */
+   The first element of the combined argument vector are arguments
+   relating to the target size ("-m64", "-m32" etc.).  These are
+   sourced from the inferior's architecture.
+
+   The second element of the combined argument vector are arguments
+   stored in the inferior DW_AT_producer section.  If these are stored
+   in the inferior (there is no guarantee that they are), they are
+   added to the vector.
+
+   The third element of the combined argument vector are argument
+   supplied by the language implementation provided by
+   compile-{lang}-support.  These contain language specific arguments.
+
+   The final element of the combined argument vector are arguments
+   supplied by the "set compile-args" command.  These are always
+   appended last so as to override any of the arguments automatically
+   generated above.  */
 
 static void
 get_args (const struct compile_instance *compiler, struct gdbarch *gdbarch,