improve docs of SetRepositorySubscription and DeleteRepositorySubscription
improve docs of SetRepositorySubscription and DeleteRepositorySubscription

Add blank lines to keep paragraphs separate.
Without a blank line, all sentences get merged into a single paragraph, which would make this less readable.
Adjust language to be more specific to go-github's methods.
Shorten it by removing some unneeded phrases like "if you would like to".
Refer to methods in go-github rather than endpoints, and be more specific about actions.
This is done to try to make it more readable and clear.

Fixes #477.
Closes #489.

Change-Id: I9e9171d0a77a1c029c804cbd50fb2e194b1c2a1d

google/go-github