Caveats with the existing TextExtracts service are documented here:
We should also surface these in the API response as warnings where possible.
 When exsentences is used in html mode a warning should be sent to the consumer "exsentences is not guaranteed to work when used in HTML mode."
 In HTML mode for all output show a warning "HTML may be malformed and/or unbalanced and may omit inline images. Use at your own risk."
= Open question
 Should we also point them to the RESTbase service in these warning messages?
 Should we add `math` to the elements that should be excluded from text extracts and check that no math tags appear inside https://en.wikipedia.org/w/api.php?format=jsonfm&action=query&prop=extracts&titles=Planck%20constant ? Alternatively we keep them - as consumers can parse them and display them if they are capable of doing so.