Doc: clarify locking requirements for ALTER TABLE ADD FOREIGN KEY.

The docs explained that a SHARE ROW EXCLUSIVE lock is needed on the
referenced table, but failed to say the same about the table being
altered.  Since the page says that ACCESS EXCLUSIVE lock is taken
unless otherwise stated, this left readers with the wrong conclusion.

Discussion: https://postgr.es/m/834603375.3470346.1586482852542@mail.yahoo.com

doc/src/sgml/ref/alter_table.sgml

@@ -142,9 +142,9 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
   <para>
    <command>ALTER TABLE</command> changes the definition of an existing table.
    There are several subforms described below. Note that the lock level required
-   may differ for each subform. An <literal>ACCESS EXCLUSIVE</literal> lock is held
+   may differ for each subform. An <literal>ACCESS EXCLUSIVE</literal> lock is
-   unless explicitly noted. When multiple subcommands are listed, the lock
+   acquired unless explicitly noted. When multiple subcommands are given, the
-   held will be the strictest one required from any subcommand.
+   lock acquired will be the strictest one required by any subcommand.
 
   <variablelist>
    <varlistentry>
@@ -387,8 +387,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     <term><literal>ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ]</literal></term>
     <listitem>
      <para>
-      This form adds a new constraint to a table using the same syntax as
+      This form adds a new constraint to a table using the same constraint
-      <xref linkend="sql-createtable"/>, plus the option <literal>NOT
+      syntax as <xref linkend="sql-createtable"/>, plus the option <literal>NOT
       VALID</literal>, which is currently only allowed for foreign key
       and CHECK constraints.
      </para>
@@ -411,9 +411,14 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      </para>
 
      <para>
-      Addition of a foreign key constraint requires a
+      Although most forms of <literal>ADD
-      <literal>SHARE ROW EXCLUSIVE</literal> lock on the referenced table,
+      <replaceable class="parameter">table_constraint</replaceable></literal>
-      in addition to the lock on the table receiving the constraint.
+      require an <literal>ACCESS EXCLUSIVE</literal> lock, <literal>ADD
+      FOREIGN KEY</literal> requires only a <literal>SHARE ROW
+      EXCLUSIVE</literal> lock.  Note that <literal>ADD FOREIGN KEY</literal>
+      also acquires a <literal>SHARE ROW EXCLUSIVE</literal> lock on the
+      referenced table, in addition to the lock on the table on which the
+      constraint is declared.
      </para>
 
      <para>