use values of 'retries' options correctly

The RE-tries option now specifies exactly that: the maximum number a
failed HTTP request is re-tried. For example a value of 2 will now
correctly stop after 3 attempts: the initial one + 2 re-tries.

The maximum wait-time now also caps at 30min and increases exponentially
for both extractor.request() and downloader.http.download().

docs/configuration.rst

@@ -300,7 +300,7 @@ extractor.*.retries
 -------------------
 =========== =====
 Type        ``integer``
-Default     ``5``
+Default     ``4``
 Description Number of times a failed HTTP request is retried before giving up.
 =========== =====
 

docs/gallery-dl.conf

@@ -154,7 +154,7 @@
         {
             "mtime": true,
             "rate": null,
-            "retries": 5,
+            "retries": 4,
             "timeout": 30.0,
             "verify": true
         },
@@ -164,7 +164,7 @@
             "format": null,
             "mtime": true,
             "rate": null,
-            "retries": 5,
+            "retries": 4,
             "timeout": 30.0,
             "verify": true
         }