bug#20784: 25.0.50; pcase documentation on t and nil

[Artur Malabarba]

I've also had reports that (pcase something ('nil 1)) complains of
invalid pattern in 24.5, while it works fine for me in 25.0.
Is there a proper way to match nil? If I just use nil, I get errors on
25, but using 'nil gives errors on 24.5.