gas: sframe: fix inaccurate function-level comment

gas/
	* gen-sframe.c (sframe_xlate_do_register): Fix code comments.

gas/gen-sframe.c

diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
index af18cba..90c8aeb 100644
--- a/gas/gen-sframe.c
+++ b/gas/gen-sframe.c
@@ -1492,15 +1492,11 @@
 /* Translate DW_CFA_register into SFrame context.
 
    This opcode indicates: Previous value of register1 is register2.  This is
-   not representable in SFrame stack trace format.  Detect the use of registers
+   not representable using FDE type SFRAME_FDE_TYPE_DEFAULT.  Hence, if
+   flexible FDE is not enabled for the ABI/arch, detect the use of registers
    interesting to SFrame (FP, RA for this opcode), and skip FDE generation
-   while warning the user.
-
-   Two exceptions apply though:
-     - for S390X, the stack offsets are used to carry register number in
-       default FDE types.  So invoke S390X specific handling.
-     - for AMD64, the flexible topmost frame encoding
-       SFRAME_FDE_TYPE_FLEX can be used for FP, RA registers.
+   while warning the user.  Same applies for SP, except that it needs special
+   handling for s390.
 
    Return SFRAME_XLATE_OK if success.  */