Improved the Documentation #413

[anumi1999]

Description

**Describe your changes in detail
The code documentation for google login is improved. Meaning of each variable is defined clearly.

Types of changes

**What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

**Go over all the following points, and put an x in all the boxes that apply.
**(If you're unsure about any of these, don't hesitate to ask. We're here to help!)

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[birm]

Interesting choice to document first, but looks reasonable to me. I'm not sure that some of the callback descriptors are necessary, but should be better than not having them. Thanks for taking the time.