refactor: add notes about grammar & rules

Signed-off-by: Victor Adossi <[email protected]>

Author: vados-cosmonic

design/mvp/WIT.md

@@ -953,7 +953,9 @@ Thus, `c` is enabled if the version is `0.2.2` or newer or the
 can be removed once producer toolchains have updated their default version to
 enable the feature by default.
 
-Specifically, the syntax for feature gates is:
+#### Feature gate syntax
+
+The grammar that governs feature gate syntax is:
 
 ```wit
 gate ::= gate-item*
@@ -969,6 +971,8 @@ feature-field ::= 'feature' '=' id
 version-field ::= 'version' '=' <valid semver>
 ```
 
+#### Rules for feature gate usage
+
 As part of WIT validation, any item that refers to another gated item must also
 be compatibly gated. For example, this is an error:
 
@@ -993,6 +997,11 @@ interface i {
 }
 ```
 
+The following rules apply to the use of feature gates:
+
+- Either `@since` *or* `@unstable` should be used, but not both (exclusive or).
+- If a package contains a feature gate, it's version must be specified (i.e. `namespace:[email protected]`)
+
 #### Scenario: Stabilization of a new feature
 
 This section lays out the basic flow and expected usage of feature gate machinery