Doco-patch for experimental::lexical_topic

'my $_' is a fatal compile-time error but warning category
experimental::lexical_topic is still mentioned in perldiag in perls 5.24
and 5.26. The attached patch just removes that paragraph from the doco.

-- 
David Cantrell | http://www.cantrell.org.uk/david

    Planckton: n, the smallest possible living thing

• Doco-patch for experimental::lexical_topic by David Cantrell
• Re: Doco-patch for experimental::lexical_topic by Father Chrysostomos