BorrowedBuf docs: Don't mention perma-unstable init feature.

We expect not to stablize the `borrowed_buf_init`. It mostly exists
as a feature because it is used in libstd but defined in libcore.

Author: briansmith

library/core/src/io/borrowed_buf.rs

@@ -17,10 +17,10 @@ use crate::ptr;
 /// ```
 ///
 /// A `BorrowedBuf` is created around some existing data (or capacity for data) via a unique reference
-/// (`&mut`). The `BorrowedBuf` can be configured (e.g., using `clear` or `set_init`), but cannot be
-/// directly written. To write into the buffer, use `unfilled` to create a `BorrowedCursor`. The cursor
-/// has write-only access to the unfilled portion of the buffer (you can think of it as a
-/// write-only iterator).
+/// (`&mut`). The `BorrowedBuf` can be configured (e.g., using `clear`), but cannot be directly
+/// written. To write into the buffer, use `unfilled` to create a `BorrowedCursor`. The cursor has
+/// write-only access to the unfilled portion of the buffer (you can think of it as a write-only
+/// iterator).
 ///
 /// The lifetime `'data` is a bound on the lifetime of the underlying data.
 pub struct BorrowedBuf<'data> {