doc: fetch: document --filter=<filter-spec> option

The `--filter=<filter-spec>` option is documented in most commands that
support it except `git fetch`.

Let's fix that and document that option using the same words already
used to document it for `git clone`.

Those words could probably be improved, but they are not wrong, so
let's just use them for now and leave improving them for future work.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: chriscool

Documentation/fetch-options.adoc

@@ -88,6 +88,16 @@ linkgit:git-config[1].
 This is incompatible with `--recurse-submodules=(yes|on-demand)` and takes
 precedence over the `fetch.output` config option.
 
+`--filter=<filter-spec>`::
+	Use the partial clone feature and request that the server sends
+	a subset of reachable objects according to a given object filter.
+	When using `--filter`, the supplied _<filter-spec>_ is used for
+	the partial fetch. For example, `--filter=blob:none` will filter
+	out all blobs (file contents) until needed by Git. Also,
+	`--filter=blob:limit=<size>` will filter out all blobs of size
+	at least _<size>_. For more details on filter specifications, see
+	the `--filter` option in linkgit:git-rev-list[1].
+
 ifndef::git-pull[]
 `--write-fetch-head`::
 `--no-write-fetch-head`::