Doc: botan_cipher_update() byte generation behavior

[reneme]

This is in response to a downstream user question. Before finalization, when passing x input bytes to botan_cipher_update it will at most generate x output bytes. However, it might generate less bytes. For instance, SIV-mode won't generate any output until the message is finalized. Typically, the number of output bytes produced will be equal to the number of input bytes consumed. Note however, that botan_cipher_update might not consume all input bytes provided.

Note that, during finalization botan_cipher_update() may generate more output bytes than input bytes. Namely, when encrypting using an AEAD, the authentication tag is added. If there's not enough space in the user-provided output buffer, BOTAN_FFI_ERROR_INSUFFICIENT_BUFFER_SPACE will be returned and output_written will be set to the number of bytes required for a successful finalization.

[coveralls]

coverage: 91.823% (+0.002%) from 91.821%
when pulling 882bf7f on Rohde-Schwarz:doc/botan_cipher_update
into 36e0acd on randombit:master.