document behavior of verify-ca for libpq compat

Author: hjr3

packages/pg-connection-string/README.md

@@ -92,7 +92,7 @@ Query parameters follow a `?` character, including the following special query p
    * `sslmode=disable` - sets `ssl` to false
    * `sslmode=no-verify`, `sslmode=prefer` - sets `ssl` to `{ rejectUnauthorized: false }`
    * `sslmode=require` - sets `ssl` to `{ rejectUnauthorized: false }` unless `sslrootcert` is specified, in which case it behaves like `verify-ca`
-   * `sslmode=verify-ca` - sets `ssl` to `{ checkServerIdentity: no-op }` (verify CA, but not server identity)
+   * `sslmode=verify-ca` - sets `ssl` to `{ checkServerIdentity: no-op }` (verify CA, but not server identity). This verifies the presented certificate against the effective CA, i.e. the one specified in sslrootcert or the system CA if sslrootcert was not specified.
    * `sslmode=verify-full` - sets `ssl` to `{}` (verify CA and server identity)
  * `sslmode=<sslmode>` when `sslcompat` is not set
    * `sslmode=disable` - sets `ssl` to false