doc/debugging_with_gdb: mention how to disable gc optimization

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6353055

doc/debugging_with_gdb.html

@@ -23,6 +23,14 @@ Pass the <code>'-s'</code> flag to the linker to omit the debug information
 (for example, <code>go build -ldflags "-s" prog.go</code>).
 </p>
 
+<p>
+The code generated by the <code>gc</code> compiler includes inlining of
+function invocations and registerization of variables. These optimizations
+can sometimes make debugging with <code>gdb</code> harder. To disable them
+when debugging, pass the flags <code>-gcflags "-N -l"</code> to the
+<a href="/cmd/go"><code>go</code></a> command used to build the code being
+debugged.
+</p>
 
 <h3 id="Common_Operations">Common Operations</h3>