mem.replace: Document that input/output cannot overlap

[squeek502]

This is just clarifying the current behavior of std.mem.replace.

Test case showing the behavior when input/output overlap (adapted from here):

const std = @import("std");

test "mem.replace overlap" {
    var buf: [256]u8 = undefined;
    const original = "Sweet apple cake. Sour crabapple pie.";
    const expected = "Sweet orange cake. Sour craborange pie.";
    buf[0..original.len].* = original.*;
    const str = buf[0..original.len];

    const needed_size = std.mem.replacementSize(u8, str, "apple", "orange");
    const output = buf[0..needed_size];
    _ = std.mem.replace(u8, str, "apple", "orange", output);
    try std.testing.expectEqualStrings(expected, output);
}

====== expected this output: =========
Sweet orange cake. Sour craborange pie.␃

======== instead found this: =========
Sweet orangeeeeeeeeeeeeeeeeeeeeeeeeeee�␃

======================================
First difference occurs on line 1:
expected:
Sweet orange cake. Sour craborange pie.
            ^ ('\x20')
found:
Sweet orangeeeeeeeeeeeeeeeeeeeeeeeeeee�
            ^ ('\x65')

[jeffective]

is this not a usecase for the noalias keyword? Unfortunately its undocumented in the langref.