doc/languages-frameworks/dotnet: improve documenation of fetch-deps

## Generating and updating NuGet dependencies {#generating-and-updating-nuget-dependencies}

When writing a new expression, you can use the generated `fetch-deps` script to initialise the lockfile.

After creating a blank `deps.nix` and pointing `nugetDeps` to it,

After setting `nugetDeps` to the desired location of the lockfile (e.g. `./deps.nix`),

build the script with `nix-build -A package.fetch-deps` and then run the result.

(When the root attr is your package, it's simply `nix-build -A fetch-deps`.)