> RDBMSs implement their own constructs. Then they add hundreds of function names, internal and user-facing. 

All of them for PostgreSQL are defined in the schema `pg_catalog`. You can access them with the `S` argument in psql. For example `\dfS` for show all system functions.

> On top of that, users go and define schema(ta)s, tables, indexes, custom functions, types, whatnot.

You can never know what users create. You'll have to check that out for each install by querying the catalog.

> Is there a truly comprehensive way to list all these 'defined words' complete with their roles, origins, comments etc. in a single huge list? 

I'm not 100% sure it's what you want, but perhaps you're looking for the [Reserved Keyword list](https://www.postgresql.org/docs/current/static/sql-keywords-appendix.html)?