Fixes gh-1400 upsert syntax

doc/sphinx/book/box/box_space.rst

@@ -555,7 +555,7 @@ A list of all ``box.space`` functions follows, then comes a list of all
 
         Parameters: :samp:`{space_object}` = an :ref:`object reference <object-reference>`;
         :samp:`{tuple_value}` (type = Lua table or scalar) =
-        field values, must be passed as a Lua table if tuple_value contains more than one field;
+        field values, must be passed as a Lua table;
         :codeitalic:`{operator, field_no, value}` (type = Lua table) = a group of arguments for each
         operation, indicating what the operation is, what field the
         operation will apply to, and what value will be applied. The

doc/sphinx/book/box/index.rst

@@ -232,7 +232,7 @@ Six examples of basic operations:
     tarantool> box.space.tester:update({999}, {{'=', 2, 'Tarantino'}})
 
     -- Upsert the tuple, changing field field[2] again.
-    -- The syntax of upsert is the same as the syntax of update,
+    -- The syntax of upsert is similar to the syntax of update,
     -- but the return value will be different.
     tarantool> box.space.tester:upsert({999}, {{'=', 2, 'Tarantism'}})