documentation: Reword introduction about external toolchains

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/buildroot.html

@@ -715,13 +715,22 @@ endif
 
     <h2 id="external_toolchain">Using an external toolchain</h2>
 
-    <p>It might be useful not to use the toolchain generated by
-    Buildroot, for example if you already have a toolchain that is known
-    to work for your specific CPU, or if the toolchain generation feature
-    of Buildroot is not sufficiently flexible for you (for example if you
-    need to generate a system with <i>glibc</i> instead of
-    <i>uClibc</i>). Buildroot supports using an <i>external
-    toolchain</i>.</p>
+    <p>Using an already existing toolchain is useful for different
+    reasons:</p>
+
+    <ul>
+      <li>you already have a toolchain that is known to work for your
+	specific CPU</li>
+      <li>you want to speed up the Buildroot build process by skipping
+      the long toolchain build part</li>
+      <li>the toolchain generation feature of Buildroot is not
+	sufficiently flexible for you (for example if you need to
+	generate a system with <i>glibc</i> instead of
+	<i>uClibc</i>)</li>
+    </ul>
+
+    <p>Buildroot supports using existing toolchains through a
+    mechanism called <i>external toolchain</i>.</p>
 
     <p>To enable the use of an external toolchain, go to the
     <code>Toolchain</code> menu, and :</p>