Make `redocly split --outDir` easier to understand

[lornajane]

Is your feature request related to a problem? Please describe.

When using the split command, the output directory --outDir should be specified and that isn't always very clear.

Describe the solution you'd like

make this more obvious in the documentation, CLI help, and when running the command itself (send a hint to stderr perhaps).

Describe alternatives you've considered

Changing the command signature for all users. Which, just after a major release where we intentionally reduced the number of positional arguments in our commands, does not seem appealing