Update doc comments and usage of DesiredCapabilities to use .copy() to avoid side effects to the global class instance.

Updated the doc comments in DesiredCapabilities, and fixed some usages as per Issue 6486

https://code.google.com/p/selenium/issues/detail?can=2&start=0&num=100&q=python&colspec=ID%20Stars%20Type%20Status%20Priority%20Milestone%20Owner%20Summary&groupby=&sort=&id=6486