uploadpack.txt: document implication of `uploadpackfilter.allow`

When `uploadpackfilter.allow` is set to `true`, it means that filters
are enabled by default except in the case where a filter is explicitly
disabled via `uploadpackilter.<filter>.allow`. This option will not only
enable the currently supported set of filters, but also any filters
which get added in the future. As such, an admin which wants to have
tight control over which filters are allowed and which aren't probably
shouldn't ever set `uploadpackfilter.allow=true`.

Amend the documentation to make the ramifications more explicit so that
admins are aware of this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/uploadpack.txt

@@ -59,7 +59,8 @@ uploadpack.allowFilter::
 
 uploadpackfilter.allow::
 	Provides a default value for unspecified object filters (see: the
-	below configuration variable).
+	below configuration variable). If set to `true`, this will also
+	enable all filters which get added in the future.
 	Defaults to `true`.
 
 uploadpackfilter.<filter>.allow::