docs: explain how to see what meson options exist

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

docs/meson.html

@@ -53,9 +53,13 @@ To see a description of your options you can run <code>meson configure</code>
 along with a build directory to view the selected options for. This will show
 your meson global arguments and project arguments, along with their defaults
 and your local settings.
+</p>
 
+<p>
 Meson does not currently support listing options before configure a build
 directory, but this feature is being discussed upstream.
+For now, the only way to see what options exist is to look at the
+<code>meson_options.txt</code> file at the root of the project.
 </p>
 
 <pre>