More clarity needed on 'camera position' - what location on what camera component do we mean?

The documentation (see 'Projection, Camera Coordinate System and Screen Window' [here](https://openexr.com/en/latest/TechnicalIntroduction.html#)) says

> We assume that a camera is located at the origin, O, of a 3D _camera coordinate system_

...but we never get specific about just what part of the camera is at the origin. Is it the image plane? Is it the entrance pupil? Is it the 'pinhole' of a pinhole camera model?

Also note that our entire definition of focus distance is this:
```
//
// focus -- the camera's focus distance, in meters
//

IMF_STD_ATTRIBUTE_DEF (focus, Focus, float)
```
and we never state from what part of the camera that focus distance is measured. SMPTE's [camdkit](https://github.com/SMPTE/ris-osvp-metadata-camdkit) measures focus distance from the image plane as do some camera vendors (cf. SMPTE RDD 55, 'Material Exchange Format — Carriage of ARRI Camera System Metadata').

Thus the clarification process will entail not just documentation cleanup in the browsable documentation but documentation cleanup in the comments of definitions in `ImfStandardAttribute.h` as well.

Start with the “Projection, Camera Coordinate System and Screen Window” section in TechnicalIntroduction.html and the focus definition comments in ImfStandardAttribute.h. Determine the intended camera reference point and the reference used for focus distance, then document those choices consistently in both locations. Done means the coordinate-system and focus-distance definitions are explicit and agreed.

Written by the indexing model from the issue text.