x/exp/slices: BinarySearchFunc slice sort undefined

[crisman]

While thinking about #59260 and #57348 (and #57433) there is now a problem with
BinarySearchFunc() (at least the documentation):

BinarySearchFunc works like BinarySearch, but uses a custom comparison
function. The slice must be sorted in increasing order, where "increasing" is
defined by cmp. cmp(a, b) is expected to return an integer comparing the two
parameters: 0 if a == b, a negative number if a < b and a positive number if
a > b.

"defined by cmp" is problematic after #57348, as to sort the slice both parameters
need to be of the same type as the slice elements. If code is using a cmp where E is
not T then the sort of x is not defined.

[gopherbot]

Change https://go.dev/cl/485995 mentions this issue: slices: update BinarySearchFunc docs for CL 464456