Document effects of [INCLUDE] and [ORDERED_INCLUDE] sections in config files

Summary

[ORDERED_INCLUDE] may include more files in order, and it may have sideeffects (e.g. [EXECUTE] section is present it is executed more times, or "&=" will add multiple module paramters)

The behavior is as expected, but this has to be documented to avoid confusion.

/cc @aknappqwt