refine logic in func 's:GetDocset()' -and- add a note about 'g:zv_get_docset_by'

[Shane-XB-Qian]

1, for example in function! s:GetDocset(file, ext, ft): l:t file is 'sh_foo_.py' l:k key is 'sh' then if match(a:{l:t}, l:k) ==# 0 was accidently matched .. but 'sh_foo_.py' is a 'py' not a 'sh' ! // adding a note to readme and doc.

2, as long as that matched -and- l:docset was not empty, then it did not need to do next/left loop anymore .. // just return it.