doc: PG 12 relnotes: update wording on truncate/vacuum item

This item prevents unauthorized locking of relations, and the previous
wording was unclear.

Reported-by: Michael Paquier

Discussion: https://postgr.es/m/20190522072651.GC1278@paquier.xyz

doc/src/sgml/release-12.sgml

@@ -1498,7 +1498,8 @@ Author: Michael Paquier <michael@paquier.xyz>
        </para>
 
        <para>
-        This prevents unauthorized locking delays.
+        This prevents unauthorized locking, which could interfere with
+        user queries.
        </para>
       </listitem>
 
@@ -1515,7 +1516,8 @@ Author: Michael Paquier <michael@paquier.xyz>
        </para>
 
        <para>
-        This prevents unauthorized locking delays.
+        This prevents unauthorized locking, which could interfere with
+        user queries.
        </para>
       </listitem>