[PATCH] Add section on function return values to CodingStyle

This patch (as776) adds a new chapter to Documentation/CodingStyle,
explaining the circumstances under which a function should return 0 for
failure and non-zero for success as opposed to a negative error code for
failure and 0 for success.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>