docs/buildroot.html: minor tweaks

- We have gconfig as well nowadays
- Variables like BUILDROOT_DL_DIR can also be set permanently in the
  environment, and not just passed to make every time.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

docs/buildroot.html

@@ -260,7 +260,7 @@ or
 
     <p>If you intend to do an offline build and just want to download
     all sources that you previously selected in the configurator
-    (<i>menuconfig</i> or <i>xconfig</i>), then issue:</p>
+    (<i>menuconfig</i>, <i>xconfig</i> or <i>gconfig</i>), then issue:</p>
 <pre>
  $ make source
 </pre>
@@ -291,7 +291,7 @@ or
     Environment variables</h3>
 
     <p>Buildroot also honors some environment variables when they are passed
-    to <code>make</code>:</p>
+    to <code>make</code> or set in the environment:</p>
     <ul>
     <li><code>HOSTCXX</code>, the host C++ compiler to use</li>
     <li><code>HOSTCC</code>, the host C compiler to use</li>