Adapt wording regarding deploy_repo and repo_preview (#2706)

Author: lgoettgens

src/deploydocs.jl

@@ -145,9 +145,9 @@ forks.
 **`branch_previews`** is the branch to which pull request previews are deployed.
 It defaults to the value of `branch`.
 
-**`repo_previews`** is the remote repository to which pull request previews are
-deployed. It defaults to the value of `repo`, and when specified manually, must
-follow its formatting scheme.
+**`repo_previews`** can be used to override the remote repository to which pull request previews are
+deployed. If this is not set, it will be the same as `deploy_repo` (if that is set) or `repo` otherwise.
+The expected format of the argument is the same as for `repo`.
 
 !!! note
     Pull requests made from forks will not have previews.