Enhance description of ut_session

man5/utmp.5

@@ -96,8 +96,8 @@ struct utmp {
        compiled 32- and 64-bit.  This allows data files and shared
        memory to be shared between 32- and 64-bit applications. */
 #if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
-    int32_t ut_session;            /* Session ID, used for
-                                      windowing */
+    int32_t ut_session;            /* Session ID (\fBgetsid\fP(2)),
+                                      used for windowing */
     struct {
         int32_t tv_sec;            /* Seconds */
         int32_t tv_usec;           /* Microseconds */
@@ -232,6 +232,11 @@ with specifications for the fields
 .IR ut_user ,
 and
 .IR ut_tv .
+POSIX.1 does not specify the lengths of the
+.I ut_line
+and
+.I ut_user
+fields.
 
 Linux defines the
 .I utmpx