Rephrase "missing descriptors" notice log to be less confusing

Some operators are confused or alarmed by these logs:

Sep 05 03:22:50.000 [notice] Our directory information is no longer up-to-date enough to build circuits: We're missing descriptors for 1/3 of our primary entry guards (total microdescriptors: 6515/6541).
Sep 05 03:22:50.000 [notice] I learned some more directory information, but not enough to build a circuit: We're missing descriptors for 1/3 of our primary entry guards (total microdescriptors: 6515/6541).

We should rephrase them or document that:

• tor tries to keep active 3 primary guards for anonymity and safety
• we'll try to get new microdescs soon
• tor usually recovers quickly from this issue