Update/fix gtk-doc support

Checklist

Reviewer's checklist:

• No obvious mistakes in the code

Hello,

this

1. updates libtasn1's gtk-doc support files from gtk-doc-tools 1.28,
2. fixes some issue in the current setup (indent fix, diverging prototypes)
3. adds some missing documentation.

env DOC_MODULE=libtasn1 gtkdoc-check

will still not succeed since this would require that everything was documented. I think doing this properly would necessitate to change the long list of #defines constants to a couple of enum the current state would become to wordy.