doc: clarify seccomp_rule_add syscall translation behavior

Refs #259.

Signed-off-by: Tudor Brindus <[email protected]>

doc/man/man3/seccomp_rule_add.3

@@ -147,7 +147,11 @@ value directly using the standard
 values, in order to ensure proper operation across multiple architectures it
 is highly recommended to use the
 .BR SCMP_SYS ()
-macro instead.  See the EXAMPLES section below.
+macro instead.  See the EXAMPLES section below.  If using
+.B __NR_syscall
+values in a context containing non-native architectures, the syscall
+corresponding to the value in the native architecture will be used in all
+non-native architectures.
 .P
 Starting with Linux v4.8, there may be a need to create a rule with a syscall
 value of -1 to allow tracing programs to skip a syscall invocation; in order