rust: update docs on legacyCargoFetcher; remove unnecessary defaults

As mentioned in #79975, the default on `legacyCargoFetcher` if left unspecified
is now `false`.

doc/languages-frameworks/rust.section.md

@@ -37,7 +37,6 @@ rustPlatform.buildRustPackage rec {
   };
 
   cargoSha256 = "17ldqr3asrdcsh4l29m3b5r37r5d0b3npq1lrgjmxb6vlx6a36qh";
-  legacyCargoFetcher = false;
 
   meta = with stdenv.lib; {
     description = "A fast line-oriented regex search tool, similar to ag and ack";
@@ -59,19 +58,9 @@ When the `Cargo.lock`, provided by upstream, is not in sync with the
 added in `cargoPatches` will also be prepended to the patches in `patches` at
 build-time.
 
-Setting `legacyCargoFetcher` to `false` enables the following behavior:
-
-1. The `Cargo.lock` file is copied into the cargo vendor directory.
-2. At buildtime, `buildRustPackage` will ensure that the `src` and `cargoSha256`
-   are consistent. This avoids errors where one but not the other is updated.
-3. The builder will compress the vendored cargo src directory into a tar.gz file
-   for storage after vendoring, and decompress it before the build. This saves
-   disk space and enables hashed mirrors for Rust dependencies.
-
-Note that this option changes the value of `cargoSha256`, so it is currently
-defaulted to `false`. When updating a Rust package, please set it to `true`;
-eventually we will default this to true and update the remaining Rust packages,
-then delete the option from all individual Rust package expressions.
+Unless `legacyCargoFetcher` is set to `true`, the fetcher will also verify that
+the `Cargo.lock` file is in sync with the `src` attribute, and will compress the
+vendor directory into a tar.gz archive.
 
 ### Building a crate for a different target