Make s390x non-clobber-only vector register support unstable

src/doc/unstable-book/src/language-features/asm-experimental-reg.md

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+# `asm_experimental_arch`
+
+The tracking issue for this feature is: [#133416]
+
+[#133416]: https://github.com/rust-lang/rust/issues/133416
+
+------------------------
+
+This tracks support for additional registers in architectures where inline assembly is already stable.
+
+## Register classes
+
+| Architecture | Register class | Registers | LLVM constraint code |
+| ------------ | -------------- | --------- | -------------------- |
+| s390x | `vreg` | `v[0-31]` | `v` |
+
+> **Notes**:
+> - s390x `vreg` is clobber-only in stable.
+
+## Register class supported types
+
+| Architecture | Register class | Target feature | Allowed types |
+| ------------ | -------------- | -------------- | ------------- |
+| s390x | `vreg` | `vector` | `i32`, `f32`, `i64`, `f64`, `i128`, `f128`, `i8x16`, `i16x8`, `i32x4`, `i64x2`, `f32x4`, `f64x2` |
+
+## Register aliases
+
+| Architecture | Base register | Aliases |
+| ------------ | ------------- | ------- |
+
+## Unsupported registers
+
+| Architecture | Unsupported register | Reason |
+| ------------ | -------------------- | ------ |
+
+## Template modifiers
+
+| Architecture | Register class | Modifier | Example output | LLVM modifier |
+| ------------ | -------------- | -------- | -------------- | ------------- |
+| s390x | `vreg` | None | `%v0` | None |