git status: document trade-offs in choosing parameters to the -u option

git status: document trade-offs in choosing parameters to the -u option

In some repostories users experience that "git status" command takes
long time.  The command spends some time searching the file system
for untracked files.

Explain the trade-off struck by the default choice of `normal` to
help users make an appropriate choice better, before talking about
the configuration variable.

Inspired by Torsten Bögershausen.

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