init/main.c: improve usability in case of init binary failure

- new Documentation/init.txt file describing various forms of failure
  trying to load the init binary after kernel bootup

- extend the init/main.c init failure message to direct to
  Documentation/init.txt

Signed-off-by: Andreas Mohr <andi@lisas.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>