docs: add Span to documentation

docs/api/locations/span.md

@@ -0,0 +1,18 @@
+# Span
+
+A `Span` is a low-level way to refer to a range using `Element` as the end points instead of a `Point` which requires the use of leaf text nodes.
+
+```typescript
+type Span = [Path, Path]
+```
+
+- [Static methods](range.md#static-methods)
+  - [Check methods](range.md#check-methods)
+
+## Static Methods
+
+### Check Methods
+
+#### `Span.isSpan(value: any) => value is Span`
+
+Check if a `value` implements the `Span` interface.