Documented 'length' function

git-svn-id: svn://svn.code.sf.net/p/magicseteditor/code/trunk@590 0fc631ac-6414-0410-93d0-97cfa31319b6

doc/function/length.txt

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+Function: length
+
+--Usage--
+> length(some_collection)
+
+Returns the number of items in a list or the number of characters in a string.
+
+Note: This function is exactly the same as [[fun:number_of_items]].
+
+--Parameters--
+! Parameter	Type				Description
+| @input@	[[type:list]] or [[type:map]] or [[type:set]] or [[type:string]]
+		  				Object to determine the number of items of.
+		  				When it is a set, returns the number of cards.
+		  				When it is a string, returns the number of characters.
+
+--Examples--
+> length(["x", "y", "z"]) == 3
+> length("pink") == 4
+
+--See also--
+| [[fun:number_of_items]]	Return the number of items in a list.