doc: Remove mention to in-place tablespaces for pg_tablespace_location()

doc: Remove mention to in-place tablespaces for pg_tablespace_location()

This paragraph has been added in the documentation by f6f0db4, but after
more discussion we found that this just makes things more confusing,
adding some cross-references between a general feature and something
only aimed at being used by developers.  The original documentation is
not wrong either, and this commit brings back this part of the docs to
the same state as before f6f0db4.

Per discussion with Kyotaro Horiguchi and Thomas Munro.

Discussion: https://postgr.es/m/CA+hUKGL2uaRKu=3+bMBpejHh4k7wqzWC05aiasTsSsHGRCWa8g@mail.gmail.com