CMake: Add description of JUCE_MODULES_ONLY to API documentation

docs/CMake API.md

@@ -224,6 +224,15 @@ plugin folders may be protected, so the build may require elevated permissions i
 installation to work correctly, or you may need to adjust the permissions of the destination
 folders.
 
+#### `JUCE_MODULES_ONLY`
+
+Only brings in targets for the built-in JUCE modules, and the `juce_add_module*` CMake functions.
+This is meant for highly custom use-cases where the `juce_add_gui_app` and `juce_add_plugin`
+functions are not required. Most importantly, the 'juceaide' helper tool is not built when this
+option is enabled, which may improve build times for established products that use other methods to
+handle plugin bundle structures, icons, plists, and so on. If this option is enabled, then
+`JUCE_ENABLE_MODULE_SOURCE_GROUPS` will have no effect.
+
 ### Functions
 
 #### `juce_add_<target>`