Docs/developer: Mention that rules will inherit its group(s) platforms

docs/manual/developer/06_contributing_with_content.md

@@ -172,13 +172,15 @@ A rule itself contains these attributes:
     under `shared/checks/oval/` and referenced in the dictionary file. It is
     possible to specify multiple platforms in the list. In that case, they are
     implicitly connected with "OR" operator.
-
+
+    Platforms from groups are inherited by rules for the whole group hierarchy. They are implicitly joined with rule's platforms using "AND" operator.
+
     The `platform` can also be a [Boolean algebra expression](https://booleanpy.readthedocs.io/en/latest/concepts.html),
     describing applicability of the rule as a combination of multiple platforms.
-    
+
     The build system recognizes `!` or `not` as "NOT" operator, `&` or `and` as "AND" operator, and `|` or `or` as "OR" operator.
     And it also allows to group and alter operator precedence with brackets: `(` and `)`.
-    
+
     For example, the expression `grub2 & !(shadow-utils | ssh)` will denote that rule is applicable for systems with *GRUB2 bootloader*,
     but only if there is no *shadow-utils* or *ssh* package installed.