@alias comments aren't visible

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### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Linux

### What is the issue affecting?

Annotations, Hover

### Expected Behaviour

a comment to an alias type should be shown as regular comment to a parameter would be shown (at least if there is no comment to the @param annotation itself). The hover message should read:

```
function f(arg: string)
@param arg — comment from alias

Unused functions.Lua Diagnostics.(unused-function)
Unused local `f`.Lua Diagnostics.(unused-local)
```

### Actual Behaviour

the comment to the alias type isn't shown. the hover message is:

```
function f(arg: string)
Unused functions.Lua Diagnostics.(unused-function)
Unused local `f`.Lua Diagnostics.(unused-local)
```

### Reproduction steps

1. cat << EOF > file.lua
---@alias t_arg string comment from alias
---@param arg t_arg
local function f(arg)
end
EOF
2. code file.lua
3. hover over the function name and read message

Reproduce the issue in file.lua using the shown @alias and @param annotations, then inspect the hover entry point for function documentation. Done means hovering over f includes the alias comment as @param arg while retaining the existing diagnostics and respecting any explicit @param comment.

Written by the indexing model from the issue text.