Add note about _object_links influencing link type in tutorial.

Author: lossyrob

docs/tutorials/adding-new-and-custom-extensions.ipynb

@@ -181,7 +181,7 @@
    "source": [
     "The `from_item` class method simply returns a new instance of the item extension given an item.\n",
     "\n",
-    "The `_object_links` class method returns the `rel` string for any links that point to STAC objects like Catalogs, Collections or Items. PySTAC needs to know which links point to STAC objects because it needs to consider them when fully resolving a STAC into in-memory objects. In a lot of cases, extensions don't add new links to STAC objects, so this is normally an empty list; however, if the extension does do this (like the `source` link in the [Label Extension](https://github.com/radiantearth/stac-spec/tree/v1.0.0-beta.2/extensions/label#links-source-imagery)), make sure to return the correct value (like the LabelItemExt is doing [here](https://github.com/azavea/pystac/blob/v0.5.0/pystac/extensions/label.py#L291-L293))."
+    "The `_object_links` class method returns the `rel` string for any links that point to STAC objects like Catalogs, Collections or Items. PySTAC needs to know which links point to STAC objects because it needs to consider them when fully resolving a STAC into in-memory objects. It also will use this information when deciding on whether to use absolute or relative HREFs for the links, based on the root catalog type. In a lot of cases, extensions don't add new links to STAC objects, so this is normally an empty list; however, if the extension does do this (like the `source` link in the [Label Extension](https://github.com/radiantearth/stac-spec/tree/v1.0.0-beta.2/extensions/label#links-source-imagery)), make sure to return the correct value (like the LabelItemExt is doing [here](https://github.com/azavea/pystac/blob/v0.5.0/pystac/extensions/label.py#L291-L293))."
    ]
   },
   {
@@ -539,4 +539,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 2
-}
+}