Vimdoc can die when referencing a function defined in the same plugin

When I put `@function(maktaba#error#Shout)` in library.vim, vimdoc dies because it can't find maktaba#error#Shout. Note that there's likely some randomness involved here depending upon which plugin file is loaded when. Vimdoc needs to load all files before outputting any, so that it's able to avoid "function not found" errors in this case.