RISC-V: Document that V registers are clobbered on syscalls

This is included in the ISA manual, but it's pretty common for bits of
the ISA manual that are actually ABI to change.  So let's document it
explicitly.
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230619190142.26498-1-palmer@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/riscv/vector.rst

@@ -130,3 +130,11 @@ processes in form of sysctl knob:
 
     Modifying the system default enablement status does not affect the enablement
     status of any existing process of thread that do not make an execve() call.
+
+3.  Vector Register State Across System Calls
+---------------------------------------------
+
+As indicated by version 1.0 of the V extension [1], vector registers are
+clobbered by system calls.
+
+1: https://github.com/riscv/riscv-v-spec/blob/master/calling-convention.adoc