Add docs

lib/modules/manager/rust-toolchain/readme.md

@@ -1,2 +1,2 @@
-This will maintain `rust-toolchain.toml` files.
-Available versions will be determined from the official Rust Github repository.
+This will maintain the [channel](https://rust-lang.github.io/rustup/overrides.html#channel) setting in [rust-toolchain.toml](https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file) files.
+Available versions will be determined from the official [Rust Github repository](https://github.com/rust-lang/rust).

lib/modules/versioning/rust-toolchain/readme.md

@@ -0,0 +1,11 @@
+The [channel](https://rust-lang.github.io/rustup/overrides.html#channel) setting in [rust-toolchain.toml](https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file) specifies which Rust toolchain to use. The value is a string in the following form:
+
+```
+<channel>       = stable|beta|nightly|<versioned>[-<prerelease>]
+<versioned>     = <major.minor>|<major.minor.patch>
+<prerelease>    = beta[.<number>]
+```
+
+Named release channels (`stable`, `beta`, `nightly`) are left unchanged, as are pre-release versions (e.g. `1.82.0-beta.1`).
+
+Versions without a patch version set (e.g. `1.82`) are similar to NPM's [tilde operator](https://docs.npmjs.com/cli/v6/using-npm/semver#tilde-ranges-123-12-1) (e.g. ~1.82.0). In other words, patch level changes are allowed. While version with a patch version set (e.g. `1.82.1`) specify a single exact version, similar to NPM's `=` operator.