core doc: modernize core.bigFileThreshold documentation

The core.bigFileThreshold documentation has been largely unchanged
since 5eef828 (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 6bf3b81 (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 5eef828.. -- '*.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/[email protected]/

Helped-by: Han Xin <[email protected]>
Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

Documentation/config/core.txt

@@ -444,17 +444,32 @@ You probably do not need to adjust this value.
 Common unit suffixes of 'k', 'm', or 'g' are supported.
 
 core.bigFileThreshold::
-	Files larger than this size are stored deflated, without
-	attempting delta compression.  Storing large files without
-	delta compression avoids excessive memory usage, at the
-	slight expense of increased disk usage. Additionally files
-	larger than this size are always treated as binary.
+	The size of files considered "big", which as discussed below
+	changes the behavior of numerous git commands, as well as how
+	such files are stored within the repository. The default is
+	512 MiB. Common unit suffixes of 'k', 'm', or 'g' are
+	supported.
 +
-Default is 512 MiB on all platforms.  This should be reasonable
-for most projects as source code and other text files can still
-be delta compressed, but larger binary media files won't be.
+Files above the configured limit will be:
 +
-Common unit suffixes of 'k', 'm', or 'g' are supported.
+* Stored deflated, without attempting delta compression.
++
+The default limit is primarily set with this use-case in mind. With it
+most projects will have their source code and other text files delta
+compressed, but not larger binary media files.
++
+Storing large files without delta compression avoids excessive memory
+usage, at the slight expense of increased disk usage.
++
+* Will be treated as if though they were labeled "binary" (see
+  linkgit:gitattributes[5]). This means that e.g. linkgit:git-log[1]
+  and linkgit:git-diff[1] will not diffs for files above this limit.
++
+* Will be generally be streamed when written, which avoids excessive
+memory usage, at the cost of some fixed overhead. Commands that make
+use of this include linkgit:git-archive[1],
+linkgit:git-fast-import[1], linkgit:git-index-pack[1] and
+linkgit:git-fsck[1].
 
 core.excludesFile::
 	Specifies the pathname to the file that contains patterns to