Manual: remove duplicate PostalAddress field for persons linked to a …

…GAP package (#5870)

* Remove duplicate PostalAddress field for persons linked to a GAP package

* Mention that mandatory components for maintainers are optional for non-maintainers

doc/ref/gappkg.xml

@@ -304,10 +304,6 @@ The following components of the record are <E>optional</E>.
   <Item>
     a string (was mandatory before &GAP; 4.14),
   </Item>
-  <Mark><C>PostalAddress</C></Mark>
-  <Item>
-    a string,
-  </Item>
   <Mark><C>Place</C></Mark>
   <Item>
     a string,
@@ -318,7 +314,7 @@ The following components of the record are <E>optional</E>.
   </Item>
   </List>
   If the <C>IsMaintainer</C> value is <K>true</K> then also one of the
-  following components is mandatory.
+  following components is mandatory, otherwise these components are optional.
   <List>
   <Mark><C>Email</C></Mark>
   <Item>