config doc: add dot-repository note

branch.<name>.remote can be set to '.' (period) as a dot repository
as part of the remote name dwimmery. Tell the reader.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
doc: command line interface (cli) dot-repository dwimmery

The Git cli will generally accept dot '.' (period) as equivalent
to the current repository when appropriate. Tell the reader of this
'do what I mean' (dwim)mery action.

Signed-off-by: Philip Oakley <philipoakley@iee.org>