Add message documentation to i18n/ directory

Based on the texts at [[Wikidata:Wikidata Lexeme Forms/English]] [1],
with some formatting improvements. Cross-references between messages (of
the form “the next message”, “the previous message”) were removed, since
on translatewiki.net messages aren’t necessarily presented in sequence.
We can add links once the messages are actually on translatewiki, where
we can preview the result of these wikitext descriptions.

[1]: https://www.wikidata.org/wiki/Wikidata:Wikidata_Lexeme_Forms/English