Clarify documentation for pretty_printer.child

An Ada pretty-printer had a bug where its 'child' method returned a
gdb.Value rather than a tuple.  Kévin suggested that the documentation
for this method could be improved to clarify this.

Reviewed-By: Kévin Le Gouguec <legouguec@adacore.com>
Approved-By: Eli Zaretskii <eliz@gnu.org>