Recover SSL_OP_CIPHER_SERVER_PREFERENCE documentation.

b9cc33a4d6d935f71c3e70af9718e716b00712e8 deleted its documentation rather than
SSL_OP_EPHEMERAL_RSA's.

Change-Id: I2e099a2dc498f145c5a3ccaac824edbda27f7e89
Reviewed-on: https://boringssl-review.googlesource.com/2407
Reviewed-by: Adam Langley <agl@google.com>