Fix a bunch of gtk doc comments

Lets make sure that at least the functions that are documented can end up in the docs. While at that fix some /** comments in regular code.

added 2 commits

• 7ce74851 - secret-store: Don't use gtk-doc style comments ('/**')
• de5b52d7 - treewide: Fix various doc comments

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changed the description

Purely cosmetic, so let's merge this.

@mohammed.sadiq if you want to look at any of the other MRs let me know and I happily let them sit for longer.

merged