company: implement --doc-buffer
That was still missing.
Wasn’t quite easy to implement, the company function one should call wants a buffer, but geiser works on a current buffer context.
So this extracts the doc-symbol buffer-filling part in its own function and reimplements company-doc-buffer
partially.
If one can extract the whole content of a buffer into a string (with formatting!), one could use company-doc-buffer
as intended.
I’m also duplicating the warning message, but haven’t found a good way to prevent that.