clarify bitmap format

Signed-off-by: William Henderson <william.henderson@nutanix.com>

docs/vfio-user.rst

diff --git a/docs/vfio-user.rst b/docs/vfio-user.rst
index a0e018c..ac57d8e 100644
--- a/docs/vfio-user.rst
+++ b/docs/vfio-user.rst
@@ -1758,7 +1758,8 @@
 the end at offset 24.
 
 The bitmap is an array of u64s that holds the output bitmap, with 1 bit
-reporting a *page_size* unit of IOVA.
+reporting a *page_size* unit of IOVA. The bits outside of the requested range
+must be zero.
 
 The mapping of IOVA to bits is given by: