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/process/coding-guidelines.rst b/docs/process/coding-guidelines.rst
index 930f76c..d524d73 100644
--- a/docs/process/coding-guidelines.rst
+++ b/docs/process/coding-guidelines.rst
@@ -7,8 +7,9 @@
 
 Some of the guidelines may also apply to other codebases.
 
-**Note:** the existing TF codebase does not necessarily comply with all the
-below guidelines but the intent is for it to do so eventually.
+.. note::
+   The existing TF codebase does not necessarily comply with all the
+   below guidelines but the intent is for it to do so eventually.
 
 Checkpatch overrides
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