Added API docs for shader.
Added API docs for shader.

RebeccaWPerry/vispy,dchilds7/Deysha-Star-Formation,bollu/vispy,sbtlaarzc/vispy,RebeccaWPerry/vispy,Eric89GXL/vispy,julienr/vispy,bollu/vispy,bollu/vispy,jay3sh/vispy,QuLogic/vispy,inclement/vispy,srinathv/vispy,ghisvail/vispy,sh4wn/vispy,jdreaver/vispy,drufat/vispy,julienr/vispy,hronoses/vispy,jdreaver/vispy,julienr/vispy,sbtlaarzc/vispy,ghisvail/vispy,sh4wn/vispy,inclement/vispy,jay3sh/vispy,hronoses/vispy,dchilds7/Deysha-Star-Formation,kkuunnddaannkk/vispy,drufat/vispy,ghisvail/vispy,Eric89GXL/vispy,hronoses/vispy,QuLogic/vispy,srinathv/vispy,jdreaver/vispy,michaelaye/vispy,jay3sh/vispy,sh4wn/vispy,michaelaye/vispy,RebeccaWPerry/vispy,kkuunnddaannkk/vispy,QuLogic/vispy,sbtlaarzc/vispy,michaelaye/vispy,drufat/vispy,inclement/vispy,kkuunnddaannkk/vispy,srinathv/vispy,dchilds7/Deysha-Star-Formation,Eric89GXL/vispy