refs composer#4199

trying to document stabilities clearly in the versions article

doc/articles/versions.md

@@ -4,13 +4,6 @@
 
 # Versions
 
-## Test version constraints
-
-You can test version constraints using [semver.mwl.be](http://semver.mwl.be).
-Fill in a package name and it will autofill the default version constraint
-which Composer would add to your `composer.json` file. You can adjust the
-version constraint and the tool will highlight all releases that match.
-
 ## Basic Constraints
 
 ### Exact
@@ -86,3 +79,33 @@ break backwards compatibility. For pre-1.0 versions it also acts with safety
 in mind and treats `^0.3` as `>=0.3.0 <0.4.0`.
 
 Example: `^1.2.3`
+
+### Stability
+
+If you are using a constraint that does not explicitly define a stability,
+Composer will default interally to `-dev` or `-stable`, depending on the
+operator(s) used. This happens transparently.
+
+If you wish to explicitly consider only the stable release in the comparison,
+add the suffix `-stable`.
+
+Examples:
+
+ Constraint         | Internally
+----------------------------------------------
+ `1.2.3`            | `=1.2.3.0-stable`
+ `>1.2`             | `>1.2.0.0-stable`
+ `>=1.2`            | `>=1.2.0.0-dev`
+ `>=1.2-stable`     | `>=1.2.0.0-stable`
+ `<1.3`             | `<1.3.0.0-dev`
+ `<=1.3`            | `<=1.3.0.0-stable`
+ `1 - 2`            | `>=1.0.0.0-dev <3.0.0.0-dev`
+ `~1.3`             | `>=1.3.0.0-dev <2.0.0.0-dev`
+ `1.4.*`            | `>=1.4.0.0-dev <1.5.0.0-dev`
+
+## Test version constraints
+
+You can test version constraints using [semver.mwl.be](http://semver.mwl.be).
+Fill in a package name and it will autofill the default version constraint
+which Composer would add to your `composer.json` file. You can adjust the
+version constraint and the tool will highlight all releases that match.