[dox] Cross-reference .merge and .multiwayMerge.

These two functions are related but play two different roles:
std.algorithm.sorting.merge takes a static number of ranges of possibly
disparate types (but compatible elements), and is useful for composing
ranges at compile-time. However, it cannot take a variable number of
ranges at runtime because all ranges must be statically known.

That latter role is played std.algorithm.setops.multiwayMerge, which
takes a range of ranges to merge, and so can merge a variable number of
ranges at runtime.  However, because of that, it cannot merge ranges of
disparate types (of compatible elements), and so is unsuitable for
compile-time range composition, unless a workaround like the range class
interface is used.

The docs for these two functions really should cross-reference each
other so that users can more easily find the correct function for their
needs.

std/algorithm/setops.d

@@ -830,6 +830,9 @@ of `ror` and discretionarily swaps and advances elements of it. If
 you want `ror` to preserve its contents after the call, you may
 want to pass a duplicate to `MultiwayMerge` (and perhaps cache the
 duplicate in between calls).
+
+See_Also: $(REF merge, std,algorithm,sorting) for an analogous function that
+    takes a static number of ranges of possibly disparate types.
  */
 struct MultiwayMerge(alias less, RangeOfRanges)
 {

std/algorithm/sorting.d

@@ -1312,6 +1312,9 @@ All of its inputs are assumed to be sorted. This can mean that inputs are
 
    If any of the inputs `rs` is infinite so is the result (`empty` being always
    `false`).
+
+See_Also: $(REF multiwayMerge, std,algorithm,setops) for an analogous function
+   that merges a dynamic number of ranges.
 */
 Merge!(less, Rs) merge(alias less = "a < b", Rs...)(Rs rs)
 if (Rs.length >= 2 &&