Rewrite four README sentences that fail the prose rules

A fresh read of README.md whole against the rules of issue 157, with every
statement checked against the code on next. Four sentences fail, all in
sections added since the last read at 742e759:

- the definition of an origin change stacked its clauses; it now says when
  one is recorded, as the code and the schema comment do
- TTL and SOA record were used without being defined; each is now defined
  on first use
- the geojs request was said to carry every address of the lookup; it
  carries the addresses the cache does not hold

Model: fable-5-1

README.md

@@ -143,9 +143,8 @@ and origin AS can have two kept routes, one or none.
   AS path, a next hop and a time. The next hop is the router the peer sends the
   prefix's traffic to. RouteWatch records the peer itself as the next hop. The
   page also lists the prefix's origin changes, newest first. An origin change is
-  an AS starting to announce the prefix, when its first peer carries it, or
+  recorded when an AS gains its first peer for the prefix, or loses its last.
-  stopping, when its last peer no longer does. The page shows at most the newest
+  The page shows at most the newest 5,000. Each row shows:
-  5,000. Each row shows:
     - the time
     - announced or withdrawn
     - the origin AS
@@ -502,11 +501,13 @@ How RouteWatch keeps the answers:
   there.
 - It writes new answers every 5 seconds to `cache.db` in the state directory
   (see Configuration), and loads them from there at startup.
-- An answer expires after the TTL of its records, the shortest one when there
+- Each DNS record carries a TTL, the time for which it may be kept. An answer
-  are several. RouteWatch keeps it for at least 60 seconds and at most 24 hours.
+  expires after the shortest TTL of its records. RouteWatch keeps it for at
-- When an address has no PTR names, that answer expires after the SOA record's
+  least 60 seconds and at most 24 hours.
-  minimum field, or the record's own TTL when that is shorter. RouteWatch keeps
+- When an address has no PTR names, the answer's SOA record says how long that
-  it for at most 1 hour.
+  answer may be kept. The answer expires after the SOA record's minimum field,
+  or the record's own TTL when that is shorter. RouteWatch keeps it for at most
+  1 hour.
 - An answer with no PTR names and no SOA record is not kept. Neither is one
   whose SOA record has a TTL or minimum field of 0.
 - A failed lookup is not kept, so the next lookup of the address asks again.
@@ -520,7 +521,8 @@ An IP lookup finds where each address is, on a best-effort basis. It uses two
 sources:
 
 - [geojs](https://www.geojs.io/) is asked first. One request carries every
-  address of the lookup, and it has 1 second to answer.
+  address of the lookup that the cache does not hold, and it has 1 second to
+  answer.
 - [DB-IP City Lite](https://db-ip.com/db/download/ip-to-city-lite) answers when
   geojs has no answer for the address. It is a database file that RouteWatch
   reads itself.