doc: pretty-formats: delineate `%<|(` parameter values

doc: pretty-formats: delineate `%<|(` parameter values

Commit a57523428b4 (pretty: support padding placeholders, %< %> and %><,
2013-04-19) introduced column width place holders. It also added
separate column position `%<|(` placeholders for display screen based
placement.

Change the display screen parameter reference from 'N' to 'M' and
corresponding descriptives to make the distinction clearer.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Junio C Hamano <gitster@pobox.com>