blob: don't recomment using git_buf_grow

We currently recommend using `git_buf_grow` in order to make a buffer
make an owned copy of the memory it points to. This is not behaviour we
should encourage, so remove this recommendation.

The function itself is not changed, as we need to remain compatible, but
it will be changed not to allow usage on borrowed buffers.

include/git2/blob.h

@@ -107,12 +107,10 @@ GIT_EXTERN(git_off_t) git_blob_rawsize(const git_blob *blob);
  * The output is written into a `git_buf` which the caller must free
  * when done (via `git_buf_free`).
  *
- * If no filters need to be applied, then the `out` buffer will just be
- * populated with a pointer to the raw content of the blob.  In that case,
- * be careful to *not* free the blob until done with the buffer.  To keep
- * the data detached from the blob, call `git_buf_grow` on the buffer
- * with a `want_size` of 0 and the buffer will be reallocated to be
- * detached from the blob.
+ * If no filters need to be applied, then the `out` buffer will just
+ * be populated with a pointer to the raw content of the blob.  In
+ * that case, be careful to *not* free the blob until done with the
+ * buffer or copy it into memory you own.
  *
  * @param out The git_buf to be filled in
  * @param blob Pointer to the blob