[PATCH,rs6000] Fix -mlong-double documentation

[Carl Love]

GCC maintianers:

The following patch improves the description of the ppc64 -mlong-double 
option.

I compiled it and tested it by hand with the command:

  gcc  --target-help

Please let me know if the patch is OK for mainline.

Thanks. 

                        Carl Love
     
------------------------------------------------------

RS6000 Fix -mlong-double documentation

gcc/ChangeLog

2020-03-06  Carl Love  <cel@us.ibm.com>
	* config/rs6000/rs6000.opt: Update the description of the
	command line option.
---
 gcc/config/rs6000/rs6000.opt | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/config/rs6000/rs6000.opt b/gcc/config/rs6000/rs6000.opt
index ed3b44a04a6..8e0ae9a0b5f 100644
--- a/gcc/config/rs6000/rs6000.opt
+++ b/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
-- 
2.17.1