Rollup merge of rust-lang#33705 - lqd:rustdoc-version-tooltip, r=GuillaumeGomez

rustdoc: Make the #[stable(since)] version attribute clearer with a tooltip

Rustdoc's new 'since' version placement only shows the version number in which the item was marked stable. This gains space but might make the meaning of this version string less clear in the docs, so I tried to bring some explicitness in a tooltip.

Author: Manishearth

src/librustdoc/html/render.rs

@@ -1566,7 +1566,8 @@ impl<'a> fmt::Display for Item<'a> {
         write!(fmt, "</span>")?; // in-band
         write!(fmt, "<span class='out-of-band'>")?;
         if let Some(version) = self.item.stable_since() {
-            write!(fmt, "<span class='since'>{}</span>", version)?;
+            write!(fmt, "<span class='since' title='Stable since Rust version {0}'>{0}</span>",
+                   version)?;
         }
         write!(fmt,
                r##"<span id='render-detail'>
@@ -2136,7 +2137,7 @@ fn render_stability_since_raw<'a>(w: &mut fmt::Formatter,
                                   containing_ver: Option<&'a str>) -> fmt::Result {
     if let Some(v) = ver {
         if containing_ver != ver && v.len() > 0 {
-            write!(w, "<div class=\"since\">{}</div>",
+            write!(w, "<div class='since' title='Stable since Rust version {0}'>{0}</div>",
                    v)?
         }
     }