Partial cleanup of script_commands.txt

- Updated examples to a more modern syntax
- Corrected examples using the wrong case for script commands (fixes
  bugreport:8192 thanks to jaBote)
  http://hercules.ws/board/tracker/issue-8192-camelcase-in-scripting-documentation/
- Reworded or rewritten some misleading/no longer correct notes to match
  the current script engine behavior.
- Marked some commands as deprecated and suggested alternatives, to
  discourage their use in new scripts (set, goto, jump_zero, menu)
- Changed use of temporary character variables into scope variables in
  examples, where appropriate.

Signed-off-by: Haru <haru@dotalux.com>