inf-child.c: Update comments.

inf-child.c: Update comments.

This file is no longer used exclusively by Unix targets anymore.

gdb/
2014-03-12  Pedro Alves  <palves@redhat.com>

	* inf-child.c: Update top comment to not mention Unix.  Add
	generic comment describing how this target is meant to be used.
	(inf_child_post_attach, inf_child_post_startup_inferior)
	(inf_child_follow_fork, inf_child_pid_to_exec_file): Don't mention
	Unix in comment.