docs: fault-injection: add requirements of error injectable functions

Add a section about the requirements of the error injectable functions and
the type of errors.

Since this section must be read before using ALLOW_ERROR_INJECTION()
macro, that section is referred from the comment of the macro too.

Link: https://lkml.kernel.org/r/167081321427.387937.15475445689482551048.stgit@devnote3
Link: https://lore.kernel.org/all/[email protected]/T/#u
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Borislav Petkov (AMD) <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Florent Revest <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: KP Singh <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt (Google) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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