Update 'postgres_ffi' module's readme file and comments.

Explain the purpose of of the 'postgres_ffi' module, explain what the
PostgreSQL control file is, and some other minor cleanup.

postgres_ffi/README

@@ -1,3 +1,25 @@
-This module contains utility functions for interacting with PostgreSQL
-file formats.
+This module contains utilities for working with PostgreSQL file
+formats. It's a collection of structs that are auto-generated from the
+PostgreSQL header files using bindgen, and Rust functions to read and
+manipulate them.
 
+There are also a bunch of constants in `pg_constants.rs` that are copied
+from various PostgreSQL headers, rather than auto-generated. They mostly
+should be auto-generated too, but that's a TODO.
+
+The PostgreSQL on-disk file format is not portable across different
+CPU architectures and operating systems. It is also subject to change
+in each major PostgreSQL version. Currently, this module is based on
+PostgreSQL v14, but in the future we will probably need a separate
+copy for each PostgreSQL version.
+
+To interact with the C structs, there is some unsafe code in this
+module. Do not copy-paste that to the rest of the codebase! Keep the
+amount of unsafe code to a minimum, and limited to this module only,
+and only where it's truly needed.
+
+TODO: Currently, there is also some code that deals with WAL records
+in pageserver/src/waldecoder.rs.  That should be moved into this
+module. The rest of the codebase should not have intimate knowledge of
+PostgreSQL file formats or WAL layout, that knowledge should be
+encapsulated in this module.