Add a section to the Enable Experimental Background Indexing document to describe the behavior without background indexing

Author: ahoppen

Documentation/Enable Experimental Background Indexing.md

@@ -1,7 +1,11 @@
-# Background Indexing
+# Enable Experimental Background Indexing
 
 Background indexing in SourceKit-LSP is available as an experimental feature. This guide shows how to set up background indexing and which caveats to expect.
 
+## Behavior Without Background Indexing
+
+By default SourceKit-LSP does not update its global index in the background or build Swift modules in the background. Thus, a lot of cross-module or global functionality is limited if the project hasn't been built recently. For example consider two modules: `Lib` and `Exec`, where `Exec` depends on `Lib`: Without background indexing, if a function is added to `Lib`, completion/jump to definition/etc in `Exec` would not be able to see that function until after a build. Background indexing solves that issue.
+
 ## Set Up
 
 1. Install a `main` or `release/6.0` Swift Development Snapshot from https://www.swift.org/install or install the [Xcode 16 beta](https://developer.apple.com/xcode/).