doc: value of SQL column used in ModulesConstraints must be a string

Documentation/SOGoInstallationGuide.asciidoc

@@ -1757,7 +1757,8 @@ Defaults to `YES` when unset.
 
 |ModulesConstraints (optional)
 |Limits the access of any module through a constraint based on a SQL
-column; must be a dictionary with keys `Mail`, and/or `Calendar`,
+column whose value is a string (e.g. `char` or `varchar` column type);
+must be a dictionary with keys `Mail`, and/or `Calendar`,
 and/or `ActiveSync` for example:
 
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