ISO Tag: doc comment PhotographicSensitivity?

[sharnoff]

After searching for the tag corresponding to the camera's ISO, the exiftool source has some enlightening comments about the PhotographicSensitivity tag:

https://github.com/exiftool/exiftool/blob/74dbab1d2766d6422bb05b033ac6634bf8d1f582/lib/Image/ExifTool/Exif.pm#L1942-L1952

(Data from my own camera(s) seems to back this up). In light of that, would it be possible to add a doc comment to indicate that Tag::PhotographicSensitivity is actually just ISO? -- the name chosen in the EXIF 2.3 spec is not very intuitive.

I'm happy to submit a PR myself, just wanted to check whether it was something you'd be in favor of.

[kamadak]

Annex G of the Exif standard describes the rationale for the use of sensitivity-related tags. Calling PhotographicSensitivity just ISO speed seems too simplistic.

That said, I agree that a doc comment would be helpful for users, so I will consider adding some explanation.

[kamadak]

I have added some comments in commit 1b44282. I hope this helps users to find sensitivity-related tags.