rev-parse: clarify documentation of $name@{upstream} syntax

"git rev-parse" interprets string in string@{upstream} as a name of
a branch not a ref. For example, refs/heads/master@{upstream} looks
for an upstream branch that is merged by git-pull to ref
refs/heads/refs/heads/master not to refs/heads/master.

However the documentation could mislead a user to believe that the
string is interpreted as ref.

Signed-off-by: Kacper Kornet <draenog@pld-linux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -88,10 +88,10 @@ some output processing may assume ref names in UTF-8.
   The construct '@\{-<n>\}' means the <n>th branch checked out
   before the current one.
 
-'<refname>@\{upstream\}', e.g. 'master@\{upstream\}', '@\{u\}'::
-  The suffix '@\{upstream\}' to a ref (short form '<refname>@\{u\}') refers to
-  the branch the ref is set to build on top of.  A missing ref defaults
-  to the current branch.
+'<branchname>@\{upstream\}', e.g. 'master@\{upstream\}', '@\{u\}'::
+  The suffix '@\{upstream\}' to a branchname (short form '<branchname>@\{u\}')
+  refers to the branch that the branch specified by branchname is set to build on
+  top of.  A missing branchname defaults to the current one.
 
 '<rev>{caret}', e.g. 'HEAD{caret}, v1.5.1{caret}0'::
   A suffix '{caret}' to a revision parameter means the first parent of