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.

Co-Authored-by Heiko Carstens <[email protected]>

Signed-off-by Eugene Syromyatnikov <[email protected]>

---
 man2/s390_sthyi.2 | 11 +++++++++++
 1 file changed, 11 insertions(+)

man2/s390_sthyi.2 +11 -0

index fb77195..d13aace 100644
@@ -53,6 +53,9 @@ capacity information.
 The
 .I resp_buffer
 argument specifies the address of a response buffer.
+When the
+.I function_code
+is 0, it has to be one page (4K) in size.
 If the system call returns 0,
 the response buffer will be filled with CPU capacity information.
 Otherwise, the response buffer's content is unchanged.

@@ -126,6 +129,14 @@ to call it.
 .PP
 For details of the STHYI instruction, see
 .UR https://www.ibm.com\:/support\:/knowledgecenter\:/SSB27U_6.3.0\:/com.ibm.zvm.v630.hcpb4\:/hcpb4sth.htm
+the documentation page
 .UE .
+.PP
+When the system call interface is used, the response buffer doesn't
+have to fulfill alignment requirements described in the STHYI
+instruction definition.
+.PP
+The kernel caches the response (for up to one second, as of Linux 4.16).
+Subsequent system call invocations may return the cached response.
 .SH SEE ALSO
 .BR syscall (2)