Clarify connections processes

source/handbook/engineering/performance/index.html.md

@@ -168,7 +168,9 @@ crude level.
 - From whitebox monitoring,
    - Of time spent on/by Rails controllers, this much is spent in the database: https://performance.gitlab.net/dashboard/db/rails-controllers?orgId=1&panelId=5&fullscreen (for a specific Rails controller / page)
    - _Global_ SQL timings: https://performance.gitlab.net/dashboard/db/transaction-overview?panelId=9&fullscreen&orgId=1&from=now-2d&to=now
-- 1 controller = 1 URL  = 1 process a.k.a. connection.
+- A single HTTP request will execute a single controller. A controller in turn
+will usually only use one available database connection, though it may use 2 if
+first a read was performed, followed by a write.
    - pgbouncer allows up to 150 concurrent PostgreSQL connections. If this limit
 is reached it will block pgbouncer connections until a PostgreSQL
  connection becomes available.