[Ada] Update "Implementation Defined Characteristics" documentation.

[Pierre-Marie de Rodat via Gcc-patches]

Ada RM M.2 gives a list of implementation-defined characteristics, each
of which is required to be documented. This requirement is addressed in
the "Implementation Defined Characteristics" section of the GNAT RM.
This section needs to be updated to reflect changes to the Ada RM M.2
over the last few years.

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

gcc/ada/

        * doc/gnat_rm/implementation_defined_characteristics.rst: Update
        this section to reflect the current version of Ada RM M.2.
        * gnat_rm.texi: Regenerate.

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