Add tip admonition for Wikidata Help:Dates

[thadguidry]

• as a result of this issue: Wikidata date upload adds calendar qualifier as URL string instead of entity link OpenRefine#6798

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[tfmorris]

Since there are only two legal values for calendar, perhaps we can just list them near line 224-224 instead of redirecting the user. The Wikidata pages says:

Currently two calendar models are supported: proleptic Gregorian calendar (Q1985727) and proleptic Julian calendar (Q1985786)

[thadguidry]

Hi @tfmorris , I thought about that. But there are Phabricator issues for Wikidata where eventually Date datatypes will get more love and also the documentation improved. There's also corner cases and special handling that is covered in their docs. So I'd rather just direct users to read up on the current Date datatypes on Wikidata's official help pages, which are actually quite hard to find just navigating around and often missed.

[wetneb]

It feels weird to me to have a "See also" block at the beginning of the subsection. If it's "see also", it makes more sense to me to have this at the end of this subsection, so that it complements the main content.