doc:  mention that INSERT can block because of unique indexes
Initial patch by David G. Johnston.
Reported-by: David G. Johnston
Discussion:
Backpatch-through: 10
@@ -75,6 +75,11 @@ INSERT INTO <replaceable class="parameter">table_name</replaceable> [ AS <replac
</para>
<para>
+ <command>INSERT</command> into tables that lack unique indexes will
+ not be blocked by concurrent activity. Tables with unique indexes
+ might block if concurrent sessions perform actions that lock or modify
+ rows matching the unique index values being inserted; the details
+ are covered in <xref linkend="index-unique-checks"/>.
<literal>ON CONFLICT</literal> can be used to specify an alternative
action to raising a unique constraint or exclusion constraint
violation error. (See <xref linkend="sql-on-conflict"
