fetch: document that pruning happens before fetching

This was changed in 10a6cc8 (fetch --prune: Run prune before
fetching, 2014-01-02), but it seems that nobody in that
discussion realized we were advertising the "after"
explicitly.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/fetch-options.txt

@@ -51,7 +51,7 @@ ifndef::git-pull[]
 
 -p::
 --prune::
-	After fetching, remove any remote-tracking references that no
+	Before fetching, remove any remote-tracking references that no
 	longer exist on the remote.  Tags are not subject to pruning
 	if they are fetched only because of the default tag
 	auto-following or due to a --tags option.  However, if tags