[webapi][XWALK-1684] Update descriptions of some manual TCs in Canvas

Impacted tests(approved): new 0, update 6, delete 0
Unit test platform: Tizen IVI
Unit test result summary: pass 6, fail 0, block 0
canvas_linestyles_linecap_001.htm
According the spec(http://www.w3.org/TR/2dcontext/#dom-context-2d-linecap), 
The square value means that a rectangle with the length of the styles lineWidth width and the width of half the styles lineWidth width, placed flat against the edge perpendicular to the direction of the line coming out of the point, must be added at each point.
So, I think, these tests don't improve  temporary.