sframe: doc: update spec for the mangled-RA bit in FRE

ChangeLog:

	* libsframe/doc/sframe-spec.texi

libsframe/doc/sframe-spec.texi

@@ -566,8 +566,8 @@ The SFrame FRE info word is a bitfield split into four parts.  From MSB to LSB:
 @multitable {Bit offset} {@code{fre_cfa_base_reg_id}} {Size of stack offsets in bytes.  Valid values}
 @headitem Bit offset @tab Name @tab Description
 @item 7
-@tab @code{unused}
-@tab Unused bit.
+@tab @code{fre_mangled_ra_p}
+@tab Indicate whether the return address is mangled with any authorization bits (signed RA).
 
 @item 5-6
 @tab @code{fre_offset_size}