docs: Add recommendations for lockfile (#3467)

I did some experiments on [this
example](https://github.com/bazelbuild/rules_rust/blob/e38fa8c2bc0990debceaf28daa4fcb2c57dcdc1c/examples/hello_world/MODULE.bazel#L25-L30)
to evaluate the `lockfile` attribute vs. using `MODULE.bazel.lock`:

I ran `bazel test //... --profile=...`. After every run, I did a `bazel
clean && bazel shutdown` to start clean.

1. Run (`MODULE.bazel.lock` file only): No `cargo-bazel` call found in
the profile generated
1. Run (add the `lockfile`, run `CARGO_BAZEL_REPIN=true bazel mod tidy`,
then `bazel test`): No `cargo-bazel` call found in the profile generated
1. Run (delete `MODULE.bazel.lock`): `cargo-bazel` was called, but cargo
splicing was very fast. The `MODULE.bazel.lock` was recreated
1. Run (delete`MODULE.bazel.lock` and run with `--lockfile_mode=off`):
`cargo-bazel` was called, similar to above

This suggests:

* If you have the `MODULE.bazel.lock` file up to date, no `cargo-bazel`
is called regardless of using the `lockfile` or not
* If the `MODULE.bazel.lock` is not tracked in VCS the `lockfile` seems
to be helpful and should be used
* Comparing the `MODULE.bazel.lock` file and the `lockfile` a bit more
in detail, shows they store similar information. The `lockfile` is a bit
more well structured json (for the rust specific content) while the
`MODULE.bazel.lock` file seems to store the entire content generated for
every crate in a single string.

Author: hofbi

crate_universe/private/crates_repository.bzl

@@ -307,7 +307,9 @@ CARGO_BAZEL_REPIN=1 CARGO_BAZEL_REPIN_ONLY=crate_index bazel sync --only=crate_i
         "lockfile": attr.label(
             doc = (
                 "The path to a file to use for reproducible renderings. " +
-                "If set, this file must exist within the workspace (but can be empty) before this rule will work."
+                "If set, this file must exist within the workspace (but can be empty) before this rule will work." +
+                "If you already have a `MODULE.bazel.lock` file, you don't need this." +
+                "If you don't have a `MODULE.bazel.lock` file, the `lockfile` will save you generation time."
             ),
         ),
         "manifests": attr.label_list(