Add documentation for title_case option

[ronaldtse]

title_case parameter has to be script-specific: it is only relevant when used with a target script that makes an upper-lower case distinction, but it will be language-independent (no matter what language it is, the script is language-agnostic, all first letters will be capitalised).

Originally posted by @chaaklau in #203 (comment)

[opoudjis]

There also needs to be documentation for downcase option I've introduced for an IPA mapping (un-ell-Grek-Latn-phonetic-1987.yaml): IPA is by definition lowercase, and there must not be any capitalisation of the transliteration based on the case of the original.