Explain properties of the lt keyword for sorting (JuliaLang#41029)

* Explain restrictions on lt functions

* Remove warn block, add note in text

base/sort.jl

@@ -669,7 +669,8 @@ Sort the vector `v` in place. [`QuickSort`](@ref) is used by default for numeric
 [`MergeSort`](@ref) is used for other arrays. You can specify an algorithm to use via the `alg`
 keyword (see [Sorting Algorithms](@ref) for available algorithms). The `by` keyword lets you provide
 a function that will be applied to each element before comparison; the `lt` keyword allows
-providing a custom "less than" function; use `rev=true` to reverse the sorting order. These
+providing a custom "less than" function (note that for every `x` and `y`, only one of `lt(x,y)`
+and `lt(y,x)` can return `true`); use `rev=true` to reverse the sorting order. These
 options are independent and can be used together in all possible combinations: if both `by`
 and `lt` are specified, the `lt` function is applied to the result of the `by` function;
 `rev=true` reverses whatever ordering specified via the `by` and `lt` keywords.