s390_sthyi.2: Some minor additions

s390_sthyi.2: Some minor additions

* man2/s390_sthyi.2
  (.SH DESCRIPTION): Document the size of the resp_buffer when
    function_code is 0.
  (.SH NOTES): Document various aspects of the current
    implementation (the lifted requirement for the response buffer
    alignment, the presence of in-kernel cache), add description
    for the documentation URL.

Coauthored-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Eugene Syromyatnikov <evgsyr@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>