x86/sgx: Add kernel-doc descriptions for params passed to vDSO user handler

x86/sgx: Add kernel-doc descriptions for params passed to vDSO user handler

Add kernel-doc markup for the register parameters passed by the vDSO blob
to the user handler to suppress build warnings, e.g.

  WARNING: arch/x86/include/uapi/asm/sgx.h:157 function parameter 'r8' not
           described in 'sgx_enclave_user_handler_t'

Call out that except for RSP, the registers are undefined on asynchronous
exits as far as the vDSO ABI is concerned.  E.g. the vDSO's exception
handler clobbers RDX, RDI, and RSI, and the kernel doesn't guarantee that
R8 or R9 will be zero (the synthetic value loaded by the CPU).

Closes: https://lore.kernel.org/all/20251106145506.145fc620@canb.auug.org.au
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251112160708.1343355-3-seanjc%40google.com