docs: Clarify a developer element is not allowed to exist multiple times

Resolves: #606

docs/xml/metainfo-component.xml

@@ -1166,14 +1166,15 @@
 		<listitem>
 		<para>
 			The <code>&lt;developer/&gt;</code> element is designed to represent the developers or project responsible for development of the project described in the metadata.
+			It must appear only once per component.
 		</para>
 		<para>
 			The element should have a <literal>id</literal> property, containing a unique ID to identify the respective developer. It is recommended to use a reverse-DNS
 			name, like <code>org.gnome</code> or <code>io.github.ximion</code>, or a Fediverse handle (like <code>@[email protected]</code>) as ID to achieve a higher chance
 			of uniqueness.
 		</para>
 		<para>
-			Every <literal>developer</literal> element must have a <literal>name</literal> tag as child, which contains a translatable name for the
+			A <literal>developer</literal> element must have a <literal>name</literal> tag as child, which contains a translatable name for the
 			respective developer. Values might be for example "The GNOME Foundation" or "The KDE Community".
 			Hyperlinks or emails must not be used in the name; if you want to link to the developer's homepage, use the <xref linkend="tag-url"/>-tag instead.
 			The <literal>name</literal> tag is translatable.