gdb, doc: describe the 'L' tracepoint action

I noticed that 'L' is a tracepoint action but it is not defined in the
document.  Add the description.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>

gdb/doc/gdb.texinfo

@@ -46071,6 +46071,9 @@ two-digit hex number in the packet; @var{len} is the number of bytes
 in the expression (and thus one-half the number of hex digits in the
 packet).
 
+@item L
+Collect static trace data.
+
 @end table
 
 Any number of actions may be packed together in a single @samp{QTDP}