docs(ios): engine: missing InstallableKeyboard and InstallableLexicalModel documentation

[mcdurdin]

https://help.keyman.com/developer/engine/iphone-and-ipad/16.0/LanguageResource/ talks about these types but they are not documented. Also, many types are either not linked up, or the links are shown as preformatted text, e.g.:

[mcdurdin]

@sgschantz assigning you, happy to discuss on prioritization.

[jahorton]

That page's "Description" was originally intended to be the documentation for those types in addition to LanguageResource. Any documentation I can think to add would essentially be heavily-WET replications of the documentation in the linked page, just with LanguageResource more explicitly and consistently being replaced by one of its two variants. (Like, find-and-replace.) If you're fine with that, I can just go that lazy route and call it a day, I guess.

I certainly won't argue the link-issue, though.

[mcdurdin]

heavily-WET

Does not really apply to documentation. That is, the documentation page should have enough detail in it for an end user to understand the purpose. If that means some duplication of content, so be it, it's not the end of the world.

More importantly though: each type needs its own documentation page, and it is acceptable to point to a parent class for more details.

[sgschantz]

moving to sprint focusing on ios