docs(ref): Remove reference to interplay of rust-version/edition

Ideally, we have rust-version checks on more fields and we shouldn't have to specify
every variant of this.

By removing this, it gives us more breathing room to expand the text
more generally.

Author: epage

src/doc/src/reference/manifest.md

@@ -176,8 +176,6 @@ rust-version = "1.56"
 The Rust version must be a bare version number with two or three components; it
 cannot include semver operators or pre-release identifiers. Compiler pre-release
 identifiers such as -nightly will be ignored while checking the Rust version.
-The `rust-version` must be equal to or newer than the version that first
-introduced the configured `edition`.
 
 The `rust-version` may be ignored using the `--ignore-rust-version` option.