Doc: elaborate on Slice section

somehow the concept of slices had less elaboration in its own dedicated section than the mentions of it elsewhere.

doc/langref.html.in

@@ -2990,15 +2990,17 @@ pub fn main() anyerror!void {
       {#header_close#}
 
       {#header_open|Slices#}
+      <p>
+      A slice is a pointer and a length. The difference between an array and
+      a slice is that the array's length is part of the type and known at
+      compile-time, whereas the slice's length is known at runtime.
+      Both can be accessed with the `len` field.
+      </p>
       {#code_begin|test_safety|test_basic_slices|index out of bounds#}
 const expect = @import("std").testing.expect;
 
 test "basic slices" {
     var array = [_]i32{ 1, 2, 3, 4 };
-    // A slice is a pointer and a length. The difference between an array and
-    // a slice is that the array's length is part of the type and known at
-    // compile-time, whereas the slice's length is known at runtime.
-    // Both can be accessed with the `len` field.
     var known_at_runtime_zero: usize = 0;
     const slice = array[known_at_runtime_zero..array.len];
     try expect(@TypeOf(slice) == []i32);