update 'git rebase' documentation

Being in the project's top directory when starting or continuing a rebase
is not necessary since 533b7039 (Allow whole-tree operations to be started
from a subdirectory, 2007-01-12).
Signed-off-by: NSZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -262,8 +262,7 @@ hook if one exists.  You can use this hook to do sanity checks and
 reject the rebase if it isn't appropriate.  Please see the template
 pre-rebase hook script for an example.
 
-You must be in the top directory of your project to start (or continue)
-a rebase.  Upon completion, <branch> will be the current branch.
+Upon completion, <branch> will be the current branch.
 
 INTERACTIVE MODE
 ----------------

git-rebase.sh

@@ -18,8 +18,7 @@ original <branch> and remove the .dotest working files, use the command
 git rebase --abort instead.
 
 Note that if <branch> is not specified on the command line, the
-currently checked out branch is used.  You must be in the top
-directory of your project to start (or continue) a rebase.
+currently checked out branch is used.
 
 Example:       git-rebase master~1 topic