Specify how ~ constraints handle pre-releases

The "next significant release" a.k.a as the tilde version constraint
would not install a pre-release (alpha, beta, RC) which is not in the same
version namespace.

But in the examples so far it was shown as `~1.2` equals `>=1.2,<2.0` which
would actually install `2.0-beta.1`, because it the pre-release is before
the `2.0` release.

See https://github.com/composer/getcomposer.org/issues/64

doc/01-basic-usage.md

@@ -82,6 +82,10 @@ including, 2.0). Since in theory there should be no backwards compatibility
 breaks until 2.0, that works well. Another way of looking at it is that using
 `~` specifies a minimum version, but allows the last digit specified to go up.
 
+> **Note:** Though `2.0-beta.1` is considered before `2.0`, a version constraint
+> like `~1.2` would not install it. So it will install every release in the same
+> *namespace* and not in the `2.0` namespace.
+
 ### Stability
 
 By default only stable releases are taken into consideration. If you would like