add dynamic tracing warning

man/man8/biosnoop.8

@@ -12,6 +12,9 @@ the I/O was first created (which usually identifies the responsible process).
 This uses in-kernel eBPF maps to cache process details (PID and comm) by I/O
 request, as well as a starting timestamp for calculating I/O latency.
 
+This works by tracing various kernel blk_*() functions using dynamic tracing,
+and will need updating to match any changes to these functions.
+
 Since this uses BPF, only the root user can use this tool.
 .SH REQUIREMENTS
 CONFIG_BPF and bcc.