Allow worlds to provide item and location descriptions (#2409)

These are displayed in the weighted options page as hoverable tooltips.

docs/world api.md

@@ -121,6 +121,38 @@ Classification is one of `LocationProgressType.DEFAULT`, `PRIORITY` or `EXCLUDED
 The Fill algorithm will force progression items to be placed at priority locations, giving a higher chance of them being
 required, and will prevent progression and useful items from being placed at excluded locations.
 
+#### Documenting Locations
+
+Worlds can optionally provide a `location_descriptions` map which contains
+human-friendly descriptions of locations or location groups. These descriptions
+will show up in location-selection options in the Weighted Options page. Extra
+indentation and single newlines will be collapsed into spaces.
+
+```python
+# Locations.py
+
+location_descriptions = {
+    "Red Potion #6": "In a secret destructible block under the second stairway",
+    "L2 Spaceship": """
+      The group of all items in the spaceship in Level 2.
+
+      This doesn't include the item on the spaceship door, since it can be
+      accessed without the Spaeship Key.
+    """
+}
+```
+
+```python
+# __init__.py
+
+from worlds.AutoWorld import World
+from .Locations import location_descriptions
+
+
+class MyGameWorld(World):
+    location_descriptions = location_descriptions
+```
+
 ### Items
 
 Items are all things that can "drop" for your game. This may be RPG items like
@@ -147,6 +179,37 @@ Other classifications include
 * `progression_skip_balancing`: the combination of `progression` and `skip_balancing`, i.e., a progression item that
   will not be moved around by progression balancing; used, e.g., for currency or tokens
 
+#### Documenting Items
+
+Worlds can optionally provide an `item_descriptions` map which contains
+human-friendly descriptions of items or item groups. These descriptions will
+show up in item-selection options in the Weighted Options page. Extra
+indentation and single newlines will be collapsed into spaces.
+
+```python
+# Items.py
+
+item_descriptions = {
+    "Red Potion": "A standard health potion",
+    "Spaceship Key": """
+      The key to the spaceship in Level 2.
+
+      This is necessary to get to the Star Realm.
+    """
+}
+```
+
+```python
+# __init__.py
+
+from worlds.AutoWorld import World
+from .Items import item_descriptions
+
+
+class MyGameWorld(World):
+    item_descriptions = item_descriptions
+```
+
 ### Events
 
 Events will mark some progress. You define an event location, an