reset: revise index refresh advice

reset: revise index refresh advice

Update the advice describing index refresh from "enumerate unstaged changes"
to "refresh the index." Describing 'refresh_index(...)' as "enumerating
unstaged changes" is not fully representative of what an index refresh is
doing; more generally, it updates the properties of index entries that are
affected by outside-of-index state, e.g. CE_UPTODATE, which is affected by
the file contents on-disk. This distinction is relevant to operations that
read the index but do not refresh first - e.g., 'git read-tree' - where a
stale index may cause incorrect behavior.

In addition to changing the advice message, use the "advise" function to
print advice.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>