docs: mention when increasing http.postBuffer is valuable

Users in a wide variety of situations find themselves with HTTP push
problems.  Oftentimes these issues are due to antivirus software,
filtering proxies, or other man-in-the-middle situations; other times,
they are due to simple unreliability of the network.

However, a common solution to HTTP push problems found online is to
increase http.postBuffer.  This works for none of the aforementioned
situations and is only useful in a small, highly restricted number of
cases: essentially, when the connection does not properly support
HTTP/1.1.

Document when raising this value is appropriate and what it actually
does, and discourage people from using it as a general solution for push
problems, since it is not effective there.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/http.txt

@@ -199,6 +199,14 @@ http.postBuffer::
 	Transfer-Encoding: chunked is used to avoid creating a
 	massive pack file locally.  Default is 1 MiB, which is
 	sufficient for most requests.
++
+Note that raising this limit is only effective for disabling chunked
+transfer encoding and therefore should be used only where the remote
+server or a proxy only supports HTTP/1.0 or is noncompliant with the
+HTTP standard.  Raising this is not, in general, an effective solution
+for most push problems, but can increase memory consumption
+significantly since the entire buffer is allocated even for small
+pushes.
 
 http.lowSpeedLimit, http.lowSpeedTime::
 	If the HTTP transfer speed is less than 'http.lowSpeedLimit'