[Docs] `Perthread` and `TextureOptBatch` docs are missing

See subject, these are missing from [the docs](https://openimageio.readthedocs.io/).

Start at the OpenImageIO documentation linked in the issue and search for `Perthread` and `TextureOptBatch`, then inspect nearby API documentation to find the appropriate section and format. Done means both named items have documented entries and the published documentation no longer omits them.

Written by the indexing model from the issue text.