Document session cache functions.

Also switch to the new variable names (SSL_CTX *ctx, SSL *ssl,
SSL_SESSION *session) for all documented functions.

Change-Id: I15e15a703b96af1727601108223c7ce3b0691f1d
Reviewed-on: https://boringssl-review.googlesource.com/5882
Reviewed-by: Adam Langley <agl@google.com>
3 files changed