docs: explain Measured Boot dependency on Trusted Boot

Change-Id: I04d9439d5967e93896dfdb0f3d7b0aec96c743f9
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 99fc21d..6f60a8e 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -463,7 +463,10 @@
    the build. The default value is 40 in debug builds and 20 in release builds.
 
 -  ``MEASURED_BOOT``: Boolean flag to include support for the Measured Boot
-   feature. If this flag is enabled ``TRUSTED_BOARD_BOOT`` must be set.
+   feature. If this flag is enabled ``TRUSTED_BOARD_BOOT`` must be set as well
+   in order to provide trust that the code taking the measurements and recording
+   them has not been tampered with.
+
    This option defaults to 0 and is an experimental feature in the stage of
    development.