Add missing option to lua-options.md

[happilyretired23]

The "lua scripts installer log debug messages" option appears on the UI in darktable 5.0, but it's not in the docs, so I added it.

This option was added to the main project over a year ago (commit 7c5907c) so perhaps this was intentional, but the worst that can happen is this PR just gets closed.

Details:

• Added a header sentence now that there are two options in this file, and hoisted the cross-reference into the header
• Documented the new option (description pulled straight from the original code commit)
• Changed the capitalization of "Lua" in the two places where it refers to the language rather than the command, to be consistent with the other "Scripting with Lua" documentation