Add ghi_clear to glossary

[RDaxini]

• Closes Standardise variable name for clearsky GHI and DNI, add to glossary #2272
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

[RDaxini]

How about a whatsnew entry along the lines of:

Added or modified definitions of the following glossary terms:
- `ghi_clear`
- ...
- ...

Whatsnew entry probably not necessary if this PR is the only glossary change, but I expect we will have further additions and/or modifications in 11.3 so maybe a single entry that lists these together would be helpful?

[cwhanse]

A single bullet in the Documentation section would be suitable to list all changes to the glossary, I think. If one change isn't enough to merit a whatsnew note, are two changes? or five? Better to list the change, I think.

[echedey-ls]

LGTM!!, minor comment below.
As an idea, you could also put the See :term: ... in irradiance.dni and irradiance.dni, which do use it (but I think it's fine as is, I don't like redirecting users for such an small description).

[RDaxini]

See :term: ... in irradiance.dni and irradiance.dni, which do use it

This should (eventually) happen #2311 so I won't do it here