clone: document --filter options

clone: document --filter options

It turns out that the "--filter=<filter-spec>" option is not
documented anywhere in the "git clone" page, and instead is
detailed carefully in "git rev-list" where it serves a
different purpose.

Add a small bit about this option in the documentation. It
would be worth some time to create a subsection in the "git clone"
documentation about partial clone as a concept and how it can be
a surprising experience. For example, "git checkout" will likely
trigger a pack download.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>