Doc: Increased `autosectionlabel_maxdepth`

This patch increases the default depth of the
`autosectionlabel_maxdepth` parameter to 2, enabling referencing
to the top-level headings across documentation by using the
`sphinx.ext.autosectionlabel` plug-in.

The patch also modifies existing offending duplicate headings,
since multiple identical labels per file are not permitted.

Change-Id: I1ad56436a44b57a44bdeefe84214fd5074aa0ce4
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>
diff --git a/docs/user_guides/tfm_secure_irq_handling.rst b/docs/user_guides/tfm_secure_irq_handling.rst
index d83ec81..970d6ba 100644
--- a/docs/user_guides/tfm_secure_irq_handling.rst
+++ b/docs/user_guides/tfm_secure_irq_handling.rst
@@ -124,9 +124,9 @@
 Implementation details
 ######################
 
-*************
-Library model
-*************
+****************************
+Library model implementation
+****************************
 
 As a result of the function call like behaviour of secure services in library
 model, some information that is critical for the SPM to keep track of partition