gprintf/gstrfuncs: Improve and port doc comments to gi-docgen

This fixes many things from the port to gi-docgen, but also improves documentation more generally.

Main improvements/fixes:

• Fix links to functions, constants, etc.
• Rewrite code syntax to work with Markdown
• Reduce indentation (do not indent by 4 to prevent code blocks)
• Remove redundant text such as "can be NULL" or "should be freed"
• Move text from large return info texts to main function text
• Add missing punctuation
• Try to improve consistency between docs for similar functions
• Annotate Strv input arguments better

Helps: #3037 (closed)

I realize this is quite big, but once you start to read a large amount of documentation in one go you realize there are some inconsistencies. This also improves that.

I have been chatting a bit on Matrix with Emmanuele, he helped me a bit on what is needed and some general guidelines, thanks!