Add descriptions to FireBird constants

Co-authored-by: Saki Takamachi <34942839+SakiTakamachi@users.noreply.github.com>

Author: Girgias

reference/pdo_firebird/pdo-firebird.xml

@@ -131,34 +131,41 @@
      <term><constant>Pdo\Firebird::TRANSACTION_ISOLATION_LEVEL</constant></term>
      <listitem>
       <simpara>
+       Set transaction isolation level. , this can be one of <constant>Pdo\Firebird::READ_COMMITTED</constant>,
+       <constant>Pdo\Firebird::REPEATABLE_READ</constant> and <constant>Pdo\Firebird::SERIALIZABLE</constant>.
       </simpara>
      </listitem>
     </varlistentry>
     <varlistentry xml:id="pdo-firebird.constants.read-committed">
      <term><constant>Pdo\Firebird::READ_COMMITTED</constant></term>
      <listitem>
       <simpara>
+       Flag to denote the transaction isolation level is READ COMMITTED.
+       This flag is the default.
       </simpara>
      </listitem>
     </varlistentry>
     <varlistentry xml:id="pdo-firebird.constants.repeatable-read">
      <term><constant>Pdo\Firebird::REPEATABLE_READ</constant></term>
      <listitem>
       <simpara>
+       Flag to denote the transaction isolation level is REPEATABLE READ.
       </simpara>
      </listitem>
     </varlistentry>
     <varlistentry xml:id="pdo-firebird.constants.serializable">
      <term><constant>Pdo\Firebird::SERIALIZABLE</constant></term>
      <listitem>
       <simpara>
+       Flag to denote the transaction isolation level is SERIALIZABLE.
       </simpara>
      </listitem>
     </varlistentry>
     <varlistentry xml:id="pdo-firebird.constants.writable-transaction">
      <term><constant>Pdo\Firebird::WRITABLE_TRANSACTION</constant></term>
      <listitem>
       <simpara>
+       Allow write operations to the database. Default is true.
       </simpara>
      </listitem>
     </varlistentry>