perf probe: Clarify x86 register naming for perf probe

Clarify how to specify x86 registers in perf probe. I recently ran into
this problem and had to figure it out from the source.
Signed-off-by: NAndi Kleen <ak@linux.intel.com>
Acked-by: NMasami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Link: http://lkml.kernel.org/r/1393596135-4227-3-git-send-email-andi@firstfloor.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-probe.txt

@@ -136,6 +136,8 @@ Each probe argument follows below syntax.
 'NAME' specifies the name of this argument (optional). You can use the name of local variable, local data structure member (e.g. var->field, var.field2), local array with fixed index (e.g. array[1], var->array[0], var->pointer[2]), or kprobe-tracer argument format (e.g. $retval, %ax, etc). Note that the name of this argument will be set as the last member name if you specify a local data structure member (e.g. field2 for 'var->field1.field2'.)
 'TYPE' casts the type of this argument (optional). If omitted, perf probe automatically set the type based on debuginfo. You can specify 'string' type only for the local variable or structure member which is an array of or a pointer to 'char' or 'unsigned char' type.
 
+On x86 systems %REG is always the short form of the register: for example %AX. %RAX or %EAX is not valid.
+
 LINE SYNTAX
 -----------
 Line range is described by following syntax.