Improve std.algorithm.find overload docs

* Group element-wise find overloads together.
* Properly document Boyer-Moore find.
Maybe, but ranges are very common in Phobos. Perhaps the module doc could be made more newbie-friendly: currently it says *"Implements algorithms oriented mainly towards processing of sequences. [snip C++ STL sentence]. Sequences processed by these functions define range-based interfaces."* It then links to std.range and a tutorial.

If this commit is causing problems, I'll remove it, because it's more important about the first two.