Merge branch 'sb/doc-upstream-branch'

* sb/doc-upstream-branch:
  Documentation: Introduce "upstream branch"

Documentation/config.txt

@@ -1215,7 +1215,7 @@ push.default::
 * `matching` push all matching branches.
   All branches having the same name in both ends are considered to be
   matching. This is the default.
-* `tracking` push the current branch to the branch it is tracking.
+* `tracking` push the current branch to its upstream branch.
 * `current` push the current branch to a branch of the same name.
 
 rebase.stat::

Documentation/glossary-content.txt

@@ -449,6 +449,12 @@ This commit is referred to as a "merge commit", or sometimes just a
 	An <<def_object,object>> which is not <<def_reachable,reachable>> from a
 	<<def_branch,branch>>, <<def_tag,tag>>, or any other reference.
 
+[[def_upstream_branch]]upstream branch::
+	The default <<def_branch,branch>> that is merged into the branch in
+	question (or the branch in question is rebased onto). It is configured
+	via branch.<name>.remote and branch.<name>.merge. If the upstream branch
+	of 'A' is 'origin/B' sometimes we say "'A' is tracking 'origin/B'".
+
 [[def_working_tree]]working tree::
 	The tree of actual checked out files.  The working tree is
 	normally equal to the <<def_HEAD,HEAD>> plus any local changes