Rename initial_ctx to session_ctx.

This makes its purpose clearer. That the session cache is based on the
initial SSL_CTX is confusing (it's a remnant of OpenSSL's backwards
session resumption ordering), but we're probably stuck with it.
Relatedly, document SSL_set_SSL_CTX better.

Change-Id: I2832efc63f6c959c5424271b365825afc7eec5e4
Reviewed-on: https://boringssl-review.googlesource.com/14204
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/tls13_client.c b/ssl/tls13_client.c
index d5f88c9..f13a4f7 100644
--- a/ssl/tls13_client.c
+++ b/ssl/tls13_client.c
@@ -258,7 +258,7 @@
 
     /* Resumption incorporates fresh key material, so refresh the timeout. */
     ssl_session_renew_timeout(ssl, hs->new_session,
-                              ssl->initial_ctx->session_psk_dhe_timeout);
+                              ssl->session_ctx->session_psk_dhe_timeout);
   } else if (!ssl_get_new_session(hs, 0)) {
     ssl3_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
     return ssl_hs_error;