Fix PowerPC prototype documentation of __builtin_mtfsf (PR93570)

PR93570 reports that the documentation shows __builtin_mtfsf to
return a double, but this is incorrect.  The return signature
should be void.

2020-02-06  Bill Schmidt  <wschmidt@linux.ibm.com>

        PR target/93570
        * doc/extend.texi (Basic PowerPC Built-in Functions): Correct
	prototype for __builtin_mtfsf.

gcc/doc/extend.texi

@@ -17166,7 +17166,7 @@ unsigned long __builtin_ppc_mftb ();
 double __builtin_unpack_ibm128 (__ibm128, int);
 __ibm128 __builtin_pack_ibm128 (double, double);
 double __builtin_mffs (void);
-double __builtin_mtfsf (const int, double);
+void __builtin_mtfsf (const int, double);
 void __builtin_mtfsb0 (const int);
 void __builtin_mtfsb1 (const int);
 void __builtin_set_fpscr_rn (int);