Syntax gating information is out of date

[clubby789]

rust-lang/rust@de17ec9/https://github.com/rust-lang/rust/pull/117204 updated the gate_feature_* macros, but

rustc-dev-guide/src/stabilization_guide.md

Lines 144 to 160 in 5d24aed

	### Do not require the feature-gate to use the feature
	Most importantly, remove the code which flags an error if the
	feature-gate is not present (since the feature is now considered
	stable). If the feature can be detected because it employs some
	new syntax, then a common place for that code to be is in the
	same `compiler/rustc_ast_passes/src/feature_gate.rs`.
	For example, you might see code like this:
	```rust,ignore
	gate_feature_post!(&self, pub_restricted, span,
	"`pub(restricted)` syntax is experimental");
	```
	This `gate_feature_post!` macro prints an error if the
	`pub_restricted` feature is not enabled. It is not needed
	now that `#[pub_restricted]` is stable.

still mentions gate_feature_post.

[ali90h]

Hi, I can take this.