rust: kernel: std_vendor: clarify licensing even further

Given Linus' comments on the `alloc` patch on v6, I thought it would
be a good idea to make this paragraph even more clear (i.e. that the
original code was actually under "Apache-2.0 OR MIT" -- the SPDX
identifier alone could be understood as "we decided to license this
ourselves under Apache/MIT, but it may not be the original license").

This also makes the wording more consistent with what we have for
`rust/alloc/README.md` -- they are now almost identical.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/std_vendor.rs

@@ -1,8 +1,9 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-//! The contents of this file come from the Rust standard library, hosted in the
-//! <https://github.com/rust-lang/rust> repository. For copyright details, see
-//! <https://github.com/rust-lang/rust/blob/master/COPYRIGHT>.
+//! The contents of this file come from the Rust standard library, hosted in
+//! the <https://github.com/rust-lang/rust> repository, licensed under
+//! "Apache-2.0 OR MIT" and adapted for kernel use. For copyright details,
+//! see <https://github.com/rust-lang/rust/blob/master/COPYRIGHT>.
 
 /// [`std::dbg`], but using [`pr_info`] instead of [`eprintln`].
 ///