protocol: clarify semantics of 32-bit RGBA values

This seems to be regularly causing confusion.

 protocol/river-window-management-v1.xml | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/protocol/river-window-management-v1.xml b/protocol/river-window-management-v1.xml
index baa96d7..f835971 100644
--- a/protocol/river-window-management-v1.xml
+++ b/protocol/river-window-management-v1.xml
@@ -620,6 +620,11 @@
         The color is defined by four 32-bit RGBA values. Unless specified in
         another protocol extension, the RGBA values use pre-multiplied alpha.
 
+        The valid range for the RGBA values is from 0x00000000 to 0xffffffff.
+        These values are interpreted as a percentage:
+        - 0x00000000 means 0% of the given color component
+        - 0xffffffff means 100% of the given color component
+
         Setting the edges to none or the width to 0 disables the borders.
         Setting a negative width is a protocol error.