docs: add details for client login final step

[lucab]

This tweaks the documentation on the main module, in order to add some details on the outcome of the client login final step. In particular, it clarifies the result of ClientLogin::finish() both on success and on errors and it adds some intra-crate links to the relevant structures and fields.

[facebook-github-bot]

Hi @lucab!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[lucab]

@kevinlewi thanks for maintaining this library!
I was recently learning about the protocol and after reading the crate docs I was left with some minor questions about the client-side login finish step. I hope this small PR will make it easier for other newcomers like me to navigate through the docs and the login flow.