External lib using pgrx datum #423

An external lib manipulating pgrx datum requires, obviously, pgrx as a dependency (currently "0.11.0") declared in Cargo.toml.

However compiling a PL/rust function that does includes such a lib in its [dependencies] section panics with the following error:

Generating bindings for pg15
[...]    
[[bindings_diff]] /----------------------------------------
libpgrx_cshim=/tmp/target/x86_64-unknown-linux-gnu/release/build/pgrx-pg-sys-f8dec8d02ed22f54/out/cshim/libpgrx-cshim-15.a
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /var/lib/postgresql/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pgrx-pg-config-0.11.0/src/lib.rs:222:21

The same error occurs if pgrx is directly declared in the [dependencies] section -- which isn't needed as it's already available.

Also, It would be worthwhile to mention in the Data types section of the docs that several of those types are pgrx datum.