I find the specific change proposal for ddl.sgml a bit weird, though, because this is a very introductory section, and you are referring people to pg_class (what is that?!?) for details. If we want to put something there, it should respect the order in which that chapter introduces concepts.
I started looking at this specific item and immediately got the idea to actually document in user-facing (i.e., not system catalogs) what these object categories are in which object types share the schema namespace. The "Other Object Types" section already in the DDL chapter seems to provide a near-perfect place to put this (not sure I like the word "other" there being my only complaint). The attached patch replaces Laurenz's v1, leaving the create_table changes as-is but presenting an alternative approach to introducing namespacing when we explain why schemas exist.
