corrections to units_metadata text

[JonathanGregory]

I've just read through the units_metadata text that was implemented today by PR #480 in the rendered versions, and noticed:

• An omission which is so silly that it's funny, namely that I forgot to add the new units_metadata attribute to Appendix A. 😄 My proposed text is "Specifies the interpretation (on-scale, difference or unknown) of the unit of temperature appearing in the units attribute."
• Some bolding which was included for clarity during the discussion and should have been removed.

I have prepared corrections for these in PR #495, and taken advantage of the opportunity to clarify the words in one place ("for guidance only regarding temperature: unknown, not as a CF convention") and to insert a missing comma.

Please could someone check and merge?

[larsbarring]

I have looked through the PR and agree with all suggestions. As we are many that have looked at the changes put forth in the first PR #480, we have our share in the burden (and perhaps dubious joy ;-) of the omission in Appendix A. Thanks @JonathanGregory for noting these. I will leave it at this for now to leave time and opportunity for yet another pair of eyes. Maybe @davidhassell could merge before minting 1.11.

[davidhassell]

Caught just in time - many thanks, Jonathan! It looks fine to me - merging.