clarify the meaning of the version number for accepted/removed features

compiler/rustc_feature/src/lib.rs

@@ -31,6 +31,10 @@ use std::num::NonZero;
 #[derive(Debug, Clone)]
 pub struct Feature {
     pub name: Symbol,
+    /// For unstable features: the version the feature was added in.
+    /// For accepted features: the version the feature got stabilized in.
+    /// For removed features we are inconsistent; sometimes this is the
+    /// version it got added, sometimes the version it got removed.
     pub since: &'static str,
     issue: Option<NonZero<u32>>,
 }