Help: Extend documentation of CMAKE_UNITY_BUILD variable
diff --git a/Help/variable/CMAKE_UNITY_BUILD.rst b/Help/variable/CMAKE_UNITY_BUILD.rst
index 3096954..b653881 100644
--- a/Help/variable/CMAKE_UNITY_BUILD.rst
+++ b/Help/variable/CMAKE_UNITY_BUILD.rst
@@ -1,6 +1,8 @@
CMAKE_UNITY_BUILD
-----------------
-Default value for :prop_tgt:`UNITY_BUILD` of targets.
-
-By default ``CMAKE_UNITY_BUILD`` is ``OFF``.
+Initializes the :prop_tgt:`UNITY_BUILD` target property on targets
+as they are created. Set to ``ON`` to batch compilation of multiple
+sources within each target. This feature is known as "Unity build",
+or "Jumbo build". By default this variable is not set and so does
+not enable unity builds on targets.
