doc/debugging_with_gdb: soften text about disabling optimizations

Don't suggest that it's always necessary to disable optimizations.
(The text can be misread that way, even if it's not what was meant.)

Change-Id: I9a2dff6a75ce4a3f9210cdf4f5bad6aaaeae9b29
Reviewed-on: https://go-review.googlesource.com/87018
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/debugging_with_gdb.html

@@ -63,10 +63,9 @@ Pass the <code>'-w'</code> flag to the linker to omit the debug information
 <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=all="-N -l"</code> to the
-<a href="/cmd/go"><code>go</code></a> command used to build the code being
-debugged.
+can sometimes make debugging with <code>gdb</code> harder.
+If you find that you need to disable these optimizations,
+build your program using <code>go</code> <code>build</code> <code>-gcflags=all="-N -l"</code>.
 </p>
 
 <p>