a68: document finding module exports in the manual

a68: document finding module exports in the manual

This commit contains some little updates on the ga68 user manual, on
the topic of modules, exports and the modules map.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>
gcc/algol68/ChangeLog

	* ga68.texi (Packets): Update to specify only particular programs
	and prelude packets are currently supported.
	(Writing modules): Few updates.
	(Module activation): Likewise.
	(Modules and exports): New section.