Add hint on build backend import with preview disabled

[zanieb]

See #9686

❯ uv run python -c "import uv; uv.build_sdist"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/zb/workspace/uv/python/uv/__init__.py", line 45, in __getattr__
    raise AttributeError(err)
AttributeError: Using `uv.build_sdist` is not allowed. The uv build backend requires preview mode to be 
                enabled, e.g., via the `UV_PREVIEW=1` environment variable.

❯ uv run python -c "import uv; uv.foo"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/zb/workspace/uv/python/uv/__init__.py", line 48, in __getattr__
    raise AttributeError(err)
AttributeError: module 'uv' has no attribute 'foo'

❯ uv run python -c "import uv; uv.find_uv_bin"

[konstin]

If you're hitting this branch, you need UV_PREVIEW=1, that's e.g. with python -m build. For uv build, you usually want --preview which goes through the direct path, except for uv build --force-pep517, in which case you need UV_PREVIEW=1, which is why I had just put "Use UV_PREVIEW=1 and --preview" on top of the docs.

We could also set preview through PEP 517 config settings, but that seems even worse.

[zanieb]

Just to confirm, is this asking for a change to what I wrote here?

[konstin]

I was only adding context on why this preview feature is complicated beyond the usual "Add --preview to use".

[zanieb]

👍 sounds good, that makes sense to me.