Various fixups:

- Use u32 for correct parameter formatting in pageserver mgmt API
  client
- Use a correctly formatted PgVersionId (not: PgMajorVersion) in
  test_branching.py
- Add some documentation on postgres_versioninfo::PgVersionId

libs/postgres_versioninfo/src/lib.rs

@@ -15,6 +15,7 @@ pub enum PgMajorVersion {
     // !!! When you add a new PgMajorVersion, don't forget to update PgMajorVersion::ALL
 }
 
+/// A full PostgreSQL version ID, in MMmmbb numerical format (Major/minor/bugfix)
 #[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
 #[repr(transparent)]
 pub struct PgVersionId(u32);