documentation: Reword introduction about external toolchains

    <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>