Fixes gh-1427 Include box.once() into reference guide

doc/sphinx/book/box/box_introspection.rst

@@ -161,7 +161,7 @@ requests since startup, broken down by request type and network events statistic
 
 .. code-block:: tarantoolsession
 
-    tarantool> type(box.stat), type(box.stat.net) -- a virtual tables
+    tarantool> type(box.stat), type(box.stat.net) -- virtual tables
     ---
     - table
     - table

doc/sphinx/book/box/limitations.rst

@@ -84,7 +84,7 @@
 
 **Number of replicas in a cluster**
 
-    32.
+    32 (``box.schema.REPLICA_MAX``).
 
 .. _lim_sophia:
 

doc/sphinx/book/replication/index.rst

@@ -167,6 +167,8 @@ If a primary server is started with:
 then there will be lines in the log file, containing the word "relay",
 when a replica connects or disconnects.
 
+.. _preventing-duplicate-actions:
+
 =====================================================================
                     Preventing Duplicate Actions
 =====================================================================

doc/sphinx/reference/index.rst

@@ -8,12 +8,12 @@
     :maxdepth: 1
 
     box
-    box_error
     clock
     console
     crypto
     csv
     digest
+    box_error
     expirationd
     fiber
     fiber-ipc
@@ -24,6 +24,7 @@
     log
     msgpack
     net_box
+    box_once
     pickle
     shard
     socket