[Ada] Fix markup in description of implementation-defined characteristics

[Ada] Fix markup in description of implementation-defined characteristics

Fix formatting of description lists, i.e. continuation lines are now
indented like their preceding lines.

Spotted while adding description of pragma Ada_2022.

gcc/ada/

	* doc/gnat_rm/implementation_defined_characteristics.rst: Remove
	extra indent in continuation lines; indent both descriptions of
	Max_Picture_Length.
	* gnat_rm.texi: Regenerate.