Where is `pg_temp` documented in the PG manual?
It is documented at https://www.postgresql.org/docs/current/static/runtime-config-client.html


  Likewise, the current session's temporary-table schema, pg_temp_nnn, is always searched if it exists. It can be explicitly listed in the path by using the alias pg_temp. If it is not listed in the path then it is searched first (even before pg_catalog). However, the temporary schema is only searched for relation (table, view, sequence, etc) and data type names. It is never searched for function or operator names.