`LogFlushLevel` somewhat user-unfriendly

`infolog.txt` doesn't flush by default, only on logs above some level. There is a springsetting to control this level though:
https://github.com/beyond-all-reason/RecoilEngine/blob/8e9cd08bb98111b76a723b55b23954789942eddf/rts/System/LogOutput.cpp#L39-L41

But it's an `int` while the description says something about "ERROR" and "WARNING". What numerical levels are those?

Would be good if:
- [ ] you could specify a string (incl. something like "any" that doesn't correspond to a specific level and maps to INT_MIN, since you can input arbitrary level via `Spring.Log` iirc)
- [ ] the named levels were documented somewhere accessible
- [ ] `Spring.Echo` had a documented level so you could at least be sure to catch those (while not going low enough to get spammed by extra verbose logs)

Start with rts/System/LogOutput.cpp around lines 39–41 to trace how LogFlushLevel is defined and interpreted. Review the documented or exposed Spring.Log and Spring.Echo entry points, then confirm the named levels and the requested string values can be explained accessibly; done means the setting accepts the intended names and the relevant levels are documented.

Written by the indexing model from the issue text.