rs6000: Fix -mlong-double documentation

gcc/ChangeLog

2020-03-09  Carl Love  <cel@us.ibm.com>

	* config/rs6000/rs6000.opt: Update the description of the
	command line option.

gcc/config/rs6000/rs6000.opt

@@ -414,7 +414,8 @@ Warn about deprecated 'vector long ...' AltiVec type usage.
 
 mlong-double-
 Target RejectNegative Joined UInteger Var(rs6000_long_double_type_size) Save
--mlong-double-[64,128]	Specify size of long double.
+Use -mlong-double-64 for 64-bit IEEE floating point format.  Use
+-mlong-double-128 for 128-bit floating point format (either IEEE or IBM).
 
 ; This option existed in the past, but now is always on.
 mlra