Revert "(DOC-1082) (PUP-1280) runinterval can never be configured on Windows"

It turns out this is a PE-only bug, caused by a dropped commit from the /ext
directory. Related docs tweaks incoming, inc. the same change for 3.7.

This reverts commit 87b7f4487c6807b236075cab7f41a5e2f84772e5.
Update windows runinterval info in 3.7 docs
Add info about PE-specific runinterval bug to 3.6 docs