docs: give more hints about how "add -e" works

docs: give more hints about how "add -e" works

The previous text was not exactly accurate; it is OK to
change space and minus lines, but only in certain ways.

This patch takes a whole new approach, which is to describe
the sorts of conceptual operations you might want to
perform. It also includes a healthy dose of warnings about
how things can go wrong.

Since the size of the text is getting quite long, it also
splits this out into an "editing patches" section. This
makes more sense with the current structure, anyway, which
already splits out the interactive mode description.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>