add std.algorithm.sum documentation links

Improve documentation somewhat based on Mike Parker's comments:

http://forum.dlang.org/post/wwonahubwyixrseqbrpq@forum.dlang.org
LINK2 will have to be redefined. Phobos docs have lots of .html links, such as this line in std.stdio:
```
stdio.d:     * $(LINK2 std_format.html#format-string, format specifier) using
```