Removed "cannot fail" from the manual

Removed "cannot fail" from the manual

This is C we're talking about.  Functions that return void cannot fail
only if they're used correctly.  Incorrect inputs can still trigger
undefined behaviour.  In this sense, those functions _can_ fail.

Returning void should be an obvious enough hint that the function
requires no error handling. At least it is if you're familiar enough
with C. (If one is not, one is not qualified to use a crypto library in
an unsafe language.)

An unqualified "cannot fail" give any more information than `void`, and
may even mislead some users.  Better stay on the safe side.