Add let g:vimtex_doc_handlers to vimtex-latexdoc-offline help section

[Xystel]

I wanted to view the documentation offline, so I read vimtex-latexdoc-offline help section. This suggested to set keywordprg=texdoc to open the documentation pdf. But the documentation was still opened online.

After searching in the Issues I found #2300, which suggested to set let g:vimtex_doc_handlers = ['vimtex#doc#handlers#texdoc'], which enabled the offline documentation. I would expect to see this option in the vimtex-latexdoc-offline help section. Also I wonder what function the option keyworgprg has.

[lervag]

Thanks, I think I've improved the docs now. Let me know if you should disagree, and feel free to suggest further improvements.

The option keywordprg is a Vim option, see :help keywordprg for more info.