Option `--doc_out_path` no longer optional

[AndreasMatthias]

Option --doc_out_path was initially implemented to be an optional argument. But since #2821 it is mandatory. You can no longer use --doc without --doc_out_path. I.e. you get an unsatisfying error with

lua-language-server --doc=.

I'd like to fix this and make --doc_out_path optional again.

What do you think is a reasonable default path?

Originally the default path of doc.json was the log path of LuaLS (aka LOGPATH in the code).
Now I'm pondering on changing the default path of doc.json to be the current directory.
Is this reasonable? What do you think? Any objections?

[rhys-vdw]

I think current directory makes sense, but I think it should log a message that informs the user of the option so they can learn to include it.

[tomlau10]

but I think it should log a message that informs the user of the option

There will be a --help soon: #3065
though initially it will only include content from luals wiki, which maybe outdated.
For example the --num_threads (multi thread check: #2638) is not documented.

[AndreasMatthias]

I could update the --doc... related help message in #3065.
What's the best way to do so? A PR to @lewis6991 repository?