Document and consistently use "Learn more" pattern when providing links in warnings or errors

We have a handful of links out to XLSForm docs:

Line 526 in 0839531

    
           "The name column for the '{}' choice list contains these duplicates: {}. Duplicate names "

,

pyxform/pyxform/xls2json.py

Line 1246 in 0839531

    
           + " Use the max-pixels parameter to speed up submission sending and save storage space. Learn more: https://xlsform.org/#image"

,

pyxform/pyxform/survey.py

Line 1103 in d9413b5

+ "Learn more: http://xlsform.org#multiple-language-support"

It would be good to introduce more and to make sure they have a consistent, documented format.

Just thought I'd mention a minor inconsistency, which is that the first link above has a period after the URL ("Learn more: [URL].") whereas the others don't ("Learn more: [URL]"). (Though I don't think it matters for Central at the moment, since we linkify URLs for warnings but not errors.)