[doc] Update InputDialog example (#5510)
[doc] Update InputDialog example (#5510)

Make it work out of the box, add some cross-references, and update a few minor things.

In response to @dontcrash.

Cf. <https://gitter.im/koreader/koreader?at=5dac431978e4974ace8c4213>.
poire-z/koreader,Markismus/koreader,poire-z/koreader,mihailim/koreader,koreader/koreader,pazos/koreader,koreader/koreader,Frenzie/koreader,Frenzie/koreader,NiLuJe/koreader,NiLuJe/koreader,Hzj-jie/koreader,mwoz123/koreader