Document errors and async_run

Author: anarthal

include/boost/redis/connection.hpp

@@ -592,6 +592,8 @@ class basic_connection {
     * This function establishes a connection to the Redis server and keeps
     * it healthy by performing the following operations:
     *
+    *  @li For Sentinel deployments (`config::sentinel::addresses` is not empty),
+    *      contacts Sentinels to obtain the address of the configured master.
     *  @li For TCP connections, resolves the server hostname passed in
     *      @ref boost::redis::config::addr.
     *  @li Establishes a physical connection to the server. For TCP connections,

include/boost/redis/error.hpp

@@ -98,14 +98,14 @@ enum class error
    /// The configuration specified UNIX sockets with Sentinel, which is not supported.
    sentinel_unix_sockets_unsupported,
 
-   // TODO: document
+   /// No Sentinel knows about the master configured in `config::sentinel::master_name`.
    sentinel_unknown_master,
 
-   // TODO: document
+   /// No Sentinel could be contacted to obtain the address of the Redis server.
    no_sentinel_reachable,
 
-   // TODO: document
-   // TODO: should this ever be returned to the user?
+   /// The contacted server is not a master as expected.
+   /// This is likely a transient failure caused by a Sentinel failover in progress.
    role_check_failed,
 };