docs: Add better description for simplesaml.nameidattribute.
docs: Add better description for simplesaml.nameidattribute.

(Also fixes a couple of formatting errors.)

Thanks to Thijs Kinkhorst for providing this patch.
vivekrajenderan/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp