doc: Add missing values for meson options -Dcassert and -Db_coverage

The -Dcassert and -Db_coverage that can be specified to a meson command
require values after an equal sign but the documentation did not
properly reflect that.  All the other options specify the argument
values they expect, so close the gap.

Author: Jelte Fennema
Discussion: https://postgr.es/m/CAGECzQRXd1z+AoQ4tC5tqPk1_NQJohf6xwdEL=z9KgxHau2maQ@mail.gmail.com

doc/src/sgml/installation.sgml

@@ -3034,7 +3034,7 @@ ninja install
      </varlistentry>
 
      <varlistentry id="configure-cassert-meson">
-      <term><option>-Dcassert</option></term>
+      <term><option>-Dcassert={ true | false }</option></term>
       <listitem>
        <para>
         Enables <firstterm>assertion</firstterm> checks in the server, which
@@ -3075,7 +3075,7 @@ ninja install
      </varlistentry>
 
      <varlistentry id="configure-b-coverage-meson">
-      <term><option>-Db_coverage</option></term>
+      <term><option>-Db_coverage={ true | false }</option></term>
       <listitem>
        <para>
         If using GCC, all programs and libraries are compiled with