tools: Document use of -I <path> in trace, argdist

tools/argdist_example.txt

@@ -363,6 +363,7 @@ optional arguments:
                         below)
   -I header, --include header
                         additional header files to include in the BPF program
+                        as either full path, or relative to '/usr/include'
 
 Probe specifier syntax:
         {p,r,t,u}:{[library],category}:function(signature)[:type[,type...]:expr[,expr...][:filter]][#label]

tools/trace_example.txt

@@ -174,6 +174,10 @@ PID    TID    COMM         FUNC             -
 777    785    automount    SyS_nanosleep    sleep for 500000000 ns
 ^C
 
+Remember to use the -I argument include the appropriate header file. We didn't
+need to do that here because `struct timespec` is used internally by the tool,
+so it always includes this header file.
+
 
 As a final example, let's trace open syscalls for a specific process. By
 default, tracing is system-wide, but the -p switch overrides this:
@@ -225,6 +229,7 @@ optional arguments:
   -U, --user-stack      output user stack trace
   -I header, --include header
                         additional header files to include in the BPF program
+                        as either full path, or relative to '/usr/include'
 
 EXAMPLES: