Add-on parameter documentation is incomplete

The [documentation for add-on technology parameters](https://message.iiasa.ac.at/en/master/model/MESSAGE/parameter_def.html#parameters-for-the-add-on-technologies) contains the sentence fragment “The upper bound of”.

- This appears to have come in with #91 which incorporated #83; so @danielhuppmann can probably complete the sentence.
- #51 could perhaps be fixed at the same time.

Open the linked add-on technology parameter documentation and locate the sentence fragment “The upper bound of”. Read issues #91, #83, and possibly #51 for context on the intended wording and related cleanup. Done means the incomplete sentence and any agreed related documentation issue are corrected and the rendered documentation reads coherently.

Written by the indexing model from the issue text.