[Ada] Clarify protection offered by preconditions on Ada.Text_IO units

[Pierre-Marie de Rodat]

These preconditions are used in particular for analysis by SPARK tools.
It is thus important that they precisely convey which errors they allow
detecting, in callers of Ada.Text_IO. Here, Layout_Error cannot be
detected in a number of cases.

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

gcc/ada/

	* libgnat/a-textio.ads: Update top-level comment.
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