Formats a collection of TItems to a readable string like "3 colors: red, yellow, green".
Each item is converted by itemToString to a string representation whose length is restricted by itemLength and the output length by totalLength. A separator (default = ", ") is placed between the string representation.
a string describing the collection items, such as "colors", "employees" etc.; default = "items"
a lambda converting each item to its string representation.