[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
Please remove 'in the below' from all the expected statements.

Another, rectangle is not good as squash. Can you improve the test?