ssl_cache: Error renaming and document improvement

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>
diff --git a/library/ssl_cache.c b/library/ssl_cache.c
index 44dc11a..e29b0bc 100644
--- a/library/ssl_cache.c
+++ b/library/ssl_cache.c
@@ -51,7 +51,7 @@
                                 size_t session_id_len,
                                 mbedtls_ssl_cache_entry **dst)
 {
-    int ret = MBEDTLS_ERR_SSL_CACHE_NOT_FOUND;
+    int ret = MBEDTLS_ERR_SSL_CACHE_ENTRY_NOT_FOUND;
 #if defined(MBEDTLS_HAVE_TIME)
     mbedtls_time_t t = mbedtls_time(NULL);
 #endif