doc: Use proper note and warning annotations

The documentation contains plenty of notes and warnings. Enable
special rendering of these blocks by converting the note prefix
into a .. note:: annotation.

Change-Id: I34e26ca6bf313d335672ab6c2645741900338822
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/design/trusted-board-boot.rst b/docs/design/trusted-board-boot.rst
index dbe2f2a..6f648f5 100644
--- a/docs/design/trusted-board-boot.rst
+++ b/docs/design/trusted-board-boot.rst
@@ -141,8 +141,9 @@
    compared with the hash of the ROTPK read from the trusted root-key storage
    registers. If they match, the BL2 hash is read from the certificate.
 
-   Note: the matching operation is platform specific and is currently
-   unimplemented on the Arm development platforms.
+   .. note::
+      The matching operation is platform specific and is currently
+      unimplemented on the Arm development platforms.
 
 -  BL1 loads the BL2 image. Its hash is calculated and compared with the hash
    read from the certificate. Control is transferred to the BL2 image if all