[ARM] Complete legend for ARM register allocation in arm.h

2017-06-05  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * config/arm/arm.h: explain F symbol found in description of ARM
    register allocation in its legend.

From-SVN: r248880

gcc/ChangeLog

+2017-06-05  Thomas Preud'homme  <thomas.preudhomme@arm.com>
+
+	* config/arm/arm.h: explain F symbol found in description of ARM
+	register allocation in its legend.
+
 2017-06-05  Jan Hubicka  <hubicka@ucw.cz>
 
 	* config/mips/frame-header-opt.c: Include profile-count.h.

gcc/config/arm/arm.h

@@ -682,7 +682,7 @@ extern int arm_arch_cmse;
 /* Standard register usage.  */
 
 /* Register allocation in ARM Procedure Call Standard
-   (S - saved over call).
+   (S - saved over call, F - Frame-related).
 
 	r0	   *	argument word/integer result
 	r1-r3		argument word