docs(screenloads): Document span conditions for screen loads metrics (#…

…1284)

Documents the conditions for collecting span metrics to satisfy the screen loads module.

The screen loads module filters for specifically ui.load transactions and will surface data from the following spans.

src/docs/sdk/performance/modules/screen-loads.mdx

@@ -2,4 +2,29 @@
 title: 'Screen Loads Module'
 ---
 
-to be defined ...
+The Screen Loads module records metrics from spans in `ui.load` transactions using specific span operations.
+
+# Transaction Conventions
+
+## Transaction Measurements
+| Key | Unit | Description | Conditions |
+|:--|:--|:--|:--|
+| `time_to_initial_display` | milliseconds | The time it took to load the first frame | required |
+| `time_to_full_display` | milliseconds | The time it took to fully load the screen | optional, requires manual instrumentation |
+
+# Span Conventions
+
+## Span Operations
+| Span OP | Description |
+|:--|:--|
+| `db.sql.query` | A database query |
+| `db.sql.room` | A database query using the Room library on Android |
+| `db.sql.transaction` | - |
+| `db` | An operation on the database |
+| `file.read` | Reading a file from the file system |
+| `file.write` | Writing to a file on the file system |
+| `http.client` | An outgoing network request |
+| `ui.load` | An operation on a mobile UI |
+| `ui.load.full_display` | When the screen has fully completed loading |
+| `ui.load.initial_display` | When the screen has rendered its first frame |
+