doc: update c-lightning to Core Lightning almost everywhere.

Mostly comments and docs: some places are actually paths, which
I have avoided changing.  We may migrate them slowly, particularly
when they're user-visible.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/examples/cln-plugin-startup.rs

@@ -1,5 +1,5 @@
 //! This is a test plugin used to verify that we can compile and run
-//! plugins using the Rust API against c-lightning.
+//! plugins using the Rust API against Core Lightning.
 #[macro_use]
 extern crate serde_json;
 use cln_plugin::{options, Builder, Error, Plugin};