t7519-status-fsmonitor: improve comments

t7519-status-fsmonitor: improve comments

The comments for the staging/unstaging test did not accurately
describe the scenario being tested.  It is not essential that
the test files being staged/unstaged appear at the end of the
index.  All that is required is that the test files are not
flagged with CE_FSMONITOR_VALID and have a position in the
index greater than the number of entries in the index after
unstaging.

The comment for this test has been updated to be more
accurate with respect to the scenario that's being tested.

Signed-off-by: William Baker <William.Baker@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>