Name |select_certificate_cb| return values

The |select_certificate_cb| return values are somewhat confusing due
to the fact that they don't match the |cert_cb| ones, despite the
similarities between the two callbacks (they both have "certificate" in
the name! well, sort of).

This also documents the error return value (-1) which was previously
undocumented, and it expands the |SSL_CTX_set_select_certificate_cb|
documentation regarding retrial (by shamelessly copying from
|SSL_CTX_set_ticket_aead_method|).

Also updates other scattered documentation that was missed by previous
changes.

Change-Id: Ib962b31d08e6475e09954cbc3c939988b0ba13f7
Reviewed-on: https://boringssl-review.googlesource.com/14245
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_lib.c

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index d16c952..e3bcb91 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -2397,8 +2397,9 @@
 
 int SSL_is_dtls(const SSL *ssl) { return ssl->method->is_dtls; }
 
-void SSL_CTX_set_select_certificate_cb(SSL_CTX *ctx,
-                                       int (*cb)(const SSL_CLIENT_HELLO *)) {
+void SSL_CTX_set_select_certificate_cb(
+    SSL_CTX *ctx,
+    enum ssl_select_cert_result_t (*cb)(const SSL_CLIENT_HELLO *)) {
   ctx->select_certificate_cb = cb;
 }