core doc: modernize core.bigFileThreshold documentation

core doc: modernize core.bigFileThreshold documentation

The core.bigFileThreshold documentation has been largely unchanged
since 5eef828bc03 (fast-import: Stream very large blobs directly to
pack, 2010-02-01).

But since then this setting has been expanded to affect a lot more
than that description indicated. Most notably in how "git diff" treats
them, see 6bf3b813486 (diff --stat: mark any file larger than
core.bigfilethreshold binary, 2014-08-16).

In addition to that, numerous commands and APIs make use of a
streaming mode for files above this threshold.

So let's attempt to summarize 12 years of changes in behavior, which
can be seen with:

    git log --oneline -Gbig_file_thre 5eef828bc03.. -- '*.c'

To do that turn this into a bullet-point list. The summary Han Xin
produced in [1] helped a lot, but is a bit too detailed for
documentation aimed at users. Let's instead summarize how
user-observable behavior differs, and generally describe how we tend
to stream these files in various commands.

1. https://lore.kernel.org/git/20220120112114.47618-5-chiyutianyi@gmail.com/

Helped-by: Han Xin <chiyutianyi@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>