docstring not working on functions starting with dot(.)

```
.test <- function(x) {
  #' A test function
  #' @param x a number
  return(x+1)
}

test <- function(x) {
  #' A test function
  #' @param x a number
  return(x+1)
}
```

`docstring(test)` works but `docstring(.test)` throws an error:
```
The specified rdFile 'AppData\Local\Temp\RtmpUPOF2L/TempPackage/man/.test.Rd' does not exist.
```