[Ada] Include info about containers in GNAT RM Implementation Advice section

[Pierre-Marie de Rodat]

For each instance of implementation advice given in the Ada RM, the
Implementation Advice section of the GNAT RM documents whether the
advice is followed (see Ada RM M.3(1)). Such documention was missing for
the implementation advice regarding Ada.Containers and its child units.
Rectify this omission.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * doc/gnat_rm/implementation_advice.rst: Add a section for RM
        A.18 .
        * gnat_rm.texi: Regenerate.

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