SubmoduleWalk#forIndex: Suppress resource warning and update Javadoc

SubmoduleWalk#forIndex: Suppress resource warning and update Javadoc

SubmoduleWalk is auto-closeable, and Eclipse warns that is is not
managed by try-with-resource. However in this case the resource should
not be closed, because the caller needs to use it. Instead, it is the
responsibility of the caller to close it after use.

Update the Javadoc to clarify this, and suppress the warning.

Change-Id: Ib7ba349353bfd3341bdcbe4bb19abaeb9f3aeba5
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>