Document chainID attribute on PDBQTParser

[pgbarletta]

PDBQTParser now parses chainID, since PR#4284.

Sites where this will impact:

• https://userguide.mdanalysis.org/stable/formats/index.html
• https://userguide.mdanalysis.org/stable/topology_system.html
• https://userguide.mdanalysis.org/stable/formats/topology.html
• https://userguide.mdanalysis.org/stable/formats/reference/pdbqt.html

Curiously, the last one says that chainID is both parsed and written, which wasn't true before. It's also the only site that was hand-written, so that explains it.

I'll do it (I can't assign people, so I can't assign myself).

[lilyminium]

Thanks @pgbarletta! I think for the autogenerated docs, they should have already updated. Could you please check the dev versions of those links please? https://userguide.mdanalysis.org/dev/formats/index.html

[pgbarletta]

Oh, right. I was checking those on my local install which had an outdated version of MDA.