[Ada] Add comment on implementation choice for byte-packed array types

[Pierre-Marie de Rodat]

This documents the implementation choice made for byte-packed array
types, where we let the code generator deal with them if the type is
composite and use the manipulation routines of the front-end if the
type is discrete.

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

gcc/ada/

	* freeze.adb (Freeze_Array_Type): Add comment on implementation
	choice for byte-packed array types.
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