Bug 1065774 - Be more precise in PromiseFlatString docs about why assigning...

 * The only way to make one is with the function |PromiseFlat[C]String|,

 * which produce a |const| instance.  ``What if I need to keep a promise

 * around for a little while?'' you might ask.  In that case, you can keep a

 * reference, like so

 * reference, like so:

 *

 *   const nsCString& flat = PromiseFlatString(aCSubstring);

 *     // this reference holds the anonymous temporary alive, but remember,

 *     // it must _still_ have a lifetime shorter than that of |aCSubstring|

 *     // Temporaries usually die after the full expression containing the

 *     // expression that created the temporary is evaluated.  But when a

 *     // temporary is assigned to a local reference, the temporary's lifetime

 *     // is extended to the reference's lifetime (C++11 [class.temporary]p5).

 *     //

 *     // This reference holds the anonymous temporary alive.  But remember: it

 *     // must _still_ have a lifetime shorter than that of |aCSubstring|, and

 *     // |aCSubstring| must not be changed while the PromiseFlatString lives.

 *

 *  SomeOSFunction(flat.get());

 *  SomeOtherOSFunction(flat.get());