document control protocol router status format surprises when using microdescriptors

In Tor 0.3.0 (or earlier?), we made clients use microdescriptors by default. This changes the format of NS_CONTROL_PORT routerstatus lines, which breaks pytorctl, and therefore the bandwidth authorities.

Apparently, this issue can be resolved by setting UseMicrodescriptors 0 or using 0.2.9.

edit: Let's update control-spec.txt to document the current (surprising) behavior and track further behavior improvements in other tickets.