git-sparse-checkout.txt: flesh out pattern set sections a bit

git-sparse-checkout.txt: flesh out pattern set sections a bit

The "Internals -- Cone Pattern Set" section starts off discussing
patterns, despite the fact that cone mode is about avoiding the
patterns.  This made sense back when non-cone mode was the default and
we started by discussing the full pattern set, but now that we are
changing the default, it makes more sense to discuss cone-mode first and
avoid the full discussion of patterns.  Split this section into two, the
first with details about how cone mode operates, and the second
following the full pattern set section and discussing how the cone mode
patterns used under the hood relate to the full pattern set.

While at it, flesh out the "Internals -- Full Pattern Set" section a bit
to include more examples as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>