langref: docs for invalid error set cast and incorrect pointer alignment

also add detection of incorrect pointer alignment at compile-time
of pointers that were constructed with `@intToPtr`.

test/compile_errors.zig

@@ -1,6 +1,16 @@
 const tests = @import("tests.zig");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add(
+        "bad @alignCast at comptime",
+        \\comptime {
+        \\    const ptr = @intToPtr(*i32, 0x1);
+        \\    const aligned = @alignCast(4, ptr);
+        \\}
+    ,
+        ".tmp_source.zig:3:35: error: pointer address 0x1 is not aligned to 4 bytes",
+    );
+
     cases.add(
         "@ptrToInt on *void",
         \\export fn entry() bool {