loads-stores.rst: add footnote that clarifies GETPC usage

Current documentation is not too clear on the GETPC usage.
In particular, when used outside the top level helper function
it causes unexpected behavior.

Signed-off-by: Emanuele Giuseppe Esposito <e.emanuelegiuseppe@gmail.com>
Message-id: 20201015095147.1691-1-e.emanuelegiuseppe@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/devel/loads-stores.rst

@@ -93,7 +93,13 @@ guest CPU state in case of a guest CPU exception.  This is passed
 to ``cpu_restore_state()``.  Therefore the value should either be 0,
 to indicate that the guest CPU state is already synchronized, or
 the result of ``GETPC()`` from the top level ``HELPER(foo)``
-function, which is a return address into the generated code.
+function, which is a return address into the generated code [#gpc]_.
+
+.. [#gpc] Note that ``GETPC()`` should be used with great care: calling
+          it in other functions that are *not* the top level
+          ``HELPER(foo)`` will cause unexpected behavior. Instead, the
+          value of ``GETPC()`` should be read from the helper and passed
+          if needed to the functions that the helper calls.
 
 Function names follow the pattern: