Merge pull request #54 from project-tsurugi/doc_begin_session_switch_…

…epoch

datastore::switch_epochとlog_channel::begin_sessionの呼出しがオーバラップする可能性についてapi コメントに追記

include/limestone/api/log_channel.h

@@ -48,6 +48,9 @@ class log_channel {
      * @brief join a persistence session for the current epoch in this channel
      * @attention this function is not thread-safe.
      * @note the current epoch is the last epoch specified by datastore::switch_epoch()
+     * @note datastore::switch_epoch() and this function can be called simultaneously.
+     * If these functions are invoked at the same time, the result will be as if one of them was called first, 
+     * but it is indeterminate which one will take precedence.
      */
     void begin_session();