advice: update message to suggest '--sparse'

advice: update message to suggest '--sparse'

The previous changes modified the behavior of 'git add', 'git rm', and
'git mv' to not adjust paths outside the sparse-checkout cone, even if
they exist in the working tree and their cache entries lack the
SKIP_WORKTREE bit. The intention is to warn users that they are doing
something potentially dangerous. The '--sparse' option was added to each
command to allow careful users the same ability they had before.

To improve the discoverability of this new functionality, add a message
to advice.updateSparsePath that mentions the existence of the option.

The previous set of changes also modified the purpose of this message to
include possibly a list of paths instead of only a list of pathspecs.
Make the warning message more clear about this new behavior.

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