[Bitbucket Source] Fix credential documentation

[cjwooo]

Spec.json points to Bitbucket Server docs for access tokens when it should be pointing to Bitbucket Cloud docs. It should also inform the user of the following:

1. Access tokens are repo-specific, therefore are only suitable for pulling a single repository per Airbyte connection.
2. App passwords are a substitute for login passwords and should be entered into the Airbyte Source's password field, along with the Bitbucket username.