Fix LazyDoc in omnifunc completion

[zlogic]

When using the rust-analyzer LSP, omnicompletion doesn't show any documentation, only the Lazy doc text:

before.mov

This issue doesn't appear in some other LSPs, such as gopls.

With a few debug logs, I found that LspResolve() is never called: because the LspSetPopupFileType() autocommand (assigned later) has a replace: true.
The LspResolve() handler for CompleteChanged is then removed and replaced with LspSetPopupFileType().

The changes fixed the issue for me:

after.mov

[zlogic]

#697 might be the root cause.