C API: nix_init_path_string unclear

[roberth]

Describe the bug

nix_init_path_string has incomplete documentation and/or behavior problem.

Path values are internally represented by absolute paths. Probably the argument needs to be absolute too.

Also note that we plan to have paths that aren't on the system, such as lazily fetched sources with fetchTree.

A better name may be nix_init_path_from_absolute_path_string.

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