comment describing the file format, and moved to rsa-session.h.

Rev: src/nettle/examples/rsa-encrypt.c:1.3
Rev: src/nettle/examples/rsa-session.h:1.2

and decryption.

Rev: src/nettle/examples/rsa-decrypt.c:1.1
Rev: src/nettle/examples/rsa-encrypt.c:1.1
Rev: src/nettle/examples/rsa-session.h:1.1