Explanations for Bethesda shaders and interpolators - 2nd try

I added issue #7 from abies into nif.xml considering suggestion from ttl269 and neomonkeus. Abies pull request should be obsolete by this, as he wasn't around since august.

Also changed lines 2666/2667 (old version) 2705/2706 (this version) back to old syntax. They caused load errors in NifSkope 1.1.3. I think NifSkope doesn't recognize '!=0' in '((Num UV Sets &amp; 61440)!=0 || (BS Num UV Sets &amp; 61440)!=0)'

@ttl269 @throttlekitty @nexustheru @amorilia @neomonkeus