Adust documentation for new changes on push

When walking revisions upon push, Gerrit only
considers refs/heads and the target branch
and will create a new change for each commit
not contained in these.

Adjust documentation to be more clear about
this.

Change-Id: Iff04399d04b3bc773a65fcb833f7b9d396c773b3
diff --git a/Documentation/user-upload.txt b/Documentation/user-upload.txt
index 86719d2..926aa71 100644
--- a/Documentation/user-upload.txt
+++ b/Documentation/user-upload.txt
@@ -580,9 +580,10 @@
 === Selecting Merge Base
 
 By default new changes are opened only for new unique commits
-that have never before been seen by the Gerrit server. Clients
-may override that behavior and force new changes to be created
-by setting the merge base SHA-1 using the '%base' argument:
+that are not part of any branch in refs/heads or the target
+branch. Clients may override that behavior and force new
+changes to be created by setting the merge base SHA-1 using
+the '%base' argument:
 
 ----
   git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/master%base=$(git rev-parse origin/master)