Fix some documentation.

Fix some errors in the documentation of {string_writer,io}.{print,write}.

Clean up the formatting of the same documentation.

library/stream.string_writer.m:
	Fix a cut-and-paste errors: there is no string_writer.print/3
	predicate.

	s/pragma foreign_code/pragma foreign_type/ in a spot where
	the latter was meant.

	Refer to the destination stream as a string writer stream, rather
	than an output stream.

library/io.m:
	s/pragma foreign_code/pragma foreign_type/ in a spot where
	the latter was meant.