Document early callback functions.

Also added a SSL_CTX_set_select_certificate_cb setter for
select_certificate_cb so code needn't access SSL_CTX directly. Plus it
serves as a convenient anchor for the documentation.

Change-Id: I23755b910e1d77d4bea7bb9103961181dd3c5efe
Reviewed-on: https://boringssl-review.googlesource.com/6291
Reviewed-by: Adam Langley <alangley@gmail.com>
diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 7b36940..5f729e2 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -2651,6 +2651,11 @@
 
 int SSL_is_server(SSL *ssl) { return ssl->server; }
 
+void SSL_CTX_set_select_certificate_cb(
+    SSL_CTX *ctx, int (*cb)(const struct ssl_early_callback_ctx *)) {
+  ctx->select_certificate_cb = cb;
+}
+
 void SSL_CTX_set_dos_protection_cb(
     SSL_CTX *ctx, int (*cb)(const struct ssl_early_callback_ctx *)) {
   ctx->dos_protection_cb = cb;