luci-base: fix uci documentation issue (#538)

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

documentation/api/modules/luci.model.uci.html

@@ -1119,6 +1119,12 @@ Name of created section
 
  
 Set a value or create a named section. 
+ 
+When invoked with three arguments <code>config</code>, <code>sectionname</code>, <code>sectiontype</code>, 
+then a named section of the given type is created. 
+ 
+When invoked with four arguments <code>config</code>, <code>sectionname</code>, <code>optionname</code> and 
+<code>optionvalue</code> then the value of the specified option is set to the given value. 
 
 
 
@@ -1138,7 +1144,7 @@ Set a value or create a named section.
 	</li>
 	
 	<li>
-	  value: UCI value or nil if you want to create a section
+	  value: UCI value or nothing if you want to create a section
 	</li>
 	
 </ul>

modules/luci-base/luasrc/model/uci.luadoc

@@ -235,12 +235,18 @@ Saves changes made to a config to make them committable.
 ---[[
 Set a value or create a named section.
 
+When invoked with three arguments `config`, `sectionname`, `sectiontype`,
+then a named section of the given type is created.
+
+When invoked with four arguments `config`, `sectionname`, `optionname` and
+`optionvalue` then the value of the specified option is set to the given value.
+
 @class function
 @name Cursor.set
 @param config	UCI config
 @param section	UCI section name
 @param option	UCI option or UCI section type
-@param value		UCI value or nil if you want to create a section
+@param value		UCI value or nothing if you want to create a section
 @return			Boolean whether operation succeeded
 ]]