Make syncSubscription function clearer

Created by: felixfbecker

@twop expressed in https://github.com/sourcegraph/sourcegraph/pull/11033#discussion_r432609208 that he found syncSubscription hard to understand. This PR adds a more elaborate description (instead of just the one-liner) and also renames the function to disambiguate the abbreviation.

@twop - if you have any more questions on this, please ask in a comment on the diff and I'll respond by changing/extending the comments 😊