Clarified that x0 handling does not apply to c.ld and c.sd

Author: christian-herber-nxp

src/zilsd.adoc

@@ -98,10 +98,12 @@ Zclsd adds the following RV32-only instructions:
 
 === Use of x0 as operand
 
-LD and C.LD instructions with destination `x0` are processed as any other load, but the result is discarded entirely and x1 is not written.
+LD instructions with destination `x0` are processed as any other load, but the result is discarded entirely and x1 is not written.
 For C.LDSP, usage of `x0` as the destination is reserved.
 
-When using `x0` as `src` of SD, C.SD or C.SDSP, the entire 64-bit operand is zero — i.e., register `x1` is not accessed.
+If using `x0` as `src` of SD or C.SDSP, the entire 64-bit operand is zero — i.e., register `x1` is not accessed.
+
+C.LD and C.SD instructions can only use `x8-15`.
 
 === Exception Handling