Documentation: coresight: Expand branch broadcast documentation

Now that there is a way of enabling branch broadcast via perf, mention
the possible use cases and known limitations.
Signed-off-by: NJames Clark <james.clark@arm.com>
Reviewed-by: Mike Leach<mike.leach@linaro.org>
Signed-off-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20220511144601.2257870-5-james.clark@arm.com

Documentation/trace/coresight/coresight-etm4x-reference.rst

@@ -656,7 +656,18 @@ Bit assignments shown below:-
     ETM_MODE_BB
 
 **description:**
-    Set to enable branch broadcast if supported in hardware [IDR0].
+    Set to enable branch broadcast if supported in hardware [IDR0]. The primary use for this feature
+    is when code is patched dynamically at run time and the full program flow may not be able to be
+    reconstructed using only conditional branches.
+
+    There is currently no support in Perf for supplying modified binaries to the decoder, so this
+    feature is only inteded to be used for debugging purposes or with a 3rd party tool.
+
+    Choosing this option will result in a significant increase in the amount of trace generated -
+    possible danger of overflows, or fewer instructions covered. Note, that this option also
+    overrides any setting of :ref:`ETM_MODE_RETURNSTACK <coresight-return-stack>`, so where a branch
+    broadcast range overlaps a return stack range, return stacks will not be available for that
+    range.
 
 .. _coresight-cycle-accurate: