Documentation of default value for config.toml's `rust.channel` is inaccurate

[dtolnay]

On current master:

rust/config.example.toml

Lines 578 to 581 in 7c2012d

	# The "channel" for the Rust build to produce. The stable/beta channels only
	# allow using stable features, whereas the nightly and dev channels allow using
	# nightly features
	#channel = "dev"

Same thing on the 1.80.0 release tag: https://github.com/rust-lang/rust/blob/1.80.0/config.example.toml#L582

This default value of "dev" used to be accurate prior to #125181, i.e. with Rust 1.79 and older. The default value comes from this hardcoded value: https://github.com/rust-lang/rust/blob/1.79.0/src/bootstrap/src/core/config/config.rs#L1179.

But post-#125181, the default value varies by how you got your source tarball, or what git branch you are on. It is not accurate for the Rust 1.80.0 release sources to say the default value for rust.channel is "dev", as it's actually "stable" now.

Mentioning @onur-ozkan @clubby789 since you worked on the PR (which I am otherwise happy with).

The change in default also needs to be called out in src/bootstrap/src/utils/change_tracker.rs.