document the way member sources shadow workspace sources

[oconnor663]

Closes #14093.

[pirate]

is there a way to do something like this?

[tool.uv.sources]
somepackage = [
    { path = "../somepackage", editable = true },  # prefer local code dir if available
    { index = "pypi" },                            # fallback to getting it from pypi normally
]

I get a confusing error trying to do this currently:

error: Failed to parse: `pyproject.toml`
  Caused by: TOML parse error at line 81, column 9
   |
81 | somepackage = [
   |         ^
Source markers must be disjoint, but the following markers overlap: `true` and `true`.

hint: replace `true` with `python_version < '0'`.

perhaps worth documeting this in the same place? or maybe here https://docs.astral.sh/uv/concepts/indexes/#searching-across-multiple-indexes