Clarify `hub merge` behavior

Fixes #1863 [ci skip]

commands/merge.go

@@ -12,7 +12,12 @@ var cmdMerge = &Command{
 	Run:          merge,
 	GitExtension: true,
 	Usage:        "merge <PULLREQ-URL>",
-	Long: `Merge a pull request with a message like the GitHub Merge Button.
+	Long: `Merge a pull request locally with a message like the GitHub Merge Button.
+
+This creates a local merge commit in the current branch, but does not actually
+change the state of the pull request. However, the pull request will get
+auto-closed and marked as "merged" as soon as the newly created merge commit is
+pushed to the default branch of the remote repository.
 
 ## Examples:
 		$ hub merge https://github.com/jingweno/gh/pull/73

share/man/man1/hub.1.ronn

@@ -40,7 +40,7 @@ git but that are extended through hub, and custom ones that hub provides.
     Initialize a git repository and add a remote pointing to GitHub.
 
   * hub-merge(1):
-    Merge a pull request with a message like the GitHub Merge Button.
+    Merge a pull request locally with a message like the GitHub Merge Button.
 
   * hub-push(1):
     Push a git branch to each of the listed remotes.