[clang] [clang][docs] assert.h is not a good candidate for a textual header (PR #165057)

[Richard Smith via cfe-commits]

zygoloid wrote:

> This isn't a problem with `<assert.h>`.

Nonetheless I think X-macros are a better example to use here than `<assert.h>`. I'd suggest just undoing the first change (removing `assert.h` from the example `std` module) but the rest of the change LGTM.

https://github.com/llvm/llvm-project/pull/165057