[docs] add examples of how to filter manga chapters by language

(closes #599)

README.rst

@@ -130,6 +130,13 @@ Get the direct URL of an image from a site that requires authentication:
     $ gallery-dl -g -u "<username>" -p "<password>" "https://seiga.nicovideo.jp/seiga/im3211703"
 
 
+Filter manga chapters by language and chapter number:
+
+.. code:: bash
+
+    $ gallery-dl --chapter-filter "lang == 'fr' and 10 <= chapter < 20" "https://mangadex.org/title/2354/"
+
+
 | Search a remote resource for URLs and download images from them:
 | (URLs for which no extractor can be found will be silently ignored)
 

docs/configuration.rst

@@ -427,12 +427,10 @@ extractor.*.chapter-range
 -------------------------
 =========== =====
 Type        ``string``
-Description Like `image-range`__, but applies to delegated URLs
-            like manga-chapters, etc.
+Description Like `image-range <extractor.*.image-range_>`__,
+            but applies to delegated URLs like manga-chapters, etc.
 =========== =====
 
-__ `extractor.*.image-range`_
-
 
 extractor.*.image-filter
 ------------------------
@@ -452,12 +450,12 @@ extractor.*.chapter-filter
 --------------------------
 =========== =====
 Type        ``string``
-Description Like `image-filter`__, but applies to delegated URLs
-            like manga-chapters, etc.
+Example     | ``"lang == 'en'"``
+            | ``"language == 'French' and 10 <= chapter < 20"``
+Description Like `image-filter <extractor.*.image-filter_>`__,
+            but applies to delegated URLs like manga-chapters, etc.
 =========== =====
 
-__ `extractor.*.image-filter`_
-
 
 extractor.*.image-unique
 ------------------------

docs/gallery-dl-example.conf

@@ -66,6 +66,7 @@
 
         "mangadex":
         {
+            "chapter-filter": "lang == 'en'",
             "postprocessors": [{
                 "name": "zip",
                 "keep-files": false,