Document SSLPassPhraseDialog's piped input mode.

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    dialog (i.e. when you use a single Pass Phrase for all N Private Key files

    this Pass Phrase is queried only once).</p></li>

<li><code>|/path/to/program [args...]</code> 

   <p>This mode allows an external program to be used which acts as a

   pipe to a particular input device; the program is sent the standard

   prompt text used for the <code>builtin</code> mode on

   <code>stdin</code>, and is expected to write password strings on

   <code>stdout</code>.  If several passwords are needed (or an

   incorrect password is entered), additional prompt text will be

   written subsequent to the first password being returned, and more

   passwords must then be written back.</p></li>

<li><code>exec:/path/to/program</code>

    <p>

    Here an external program is configured which is called at startup for each