Mention str(<label>) feature in documentation for label (#2906)

crates/typst/src/foundations/label.rs

@@ -12,6 +12,9 @@ use crate::util::PicoStr;
 /// A labelled element can be [referenced]($ref), [queried]($query) for, and
 /// [styled]($styling) through its label.
 ///
+/// Once constructed, you can get the name of a label using
+/// [`str`]($str/#constructor).
+///
 /// # Example
 /// ```example
 /// #show <a>: set text(blue)

tests/typ/compiler/label.typ

@@ -63,3 +63,10 @@ _Visible_
 #test([Hello<hi>].label, <hi>)
 #test([#[A *B* C]<hi>].label, <hi>)
 #test([#text(red)[Hello]<hi>].label, <hi>)
+
+---
+// Test getting the name of a label.
+// Ref: false
+#test(str(<hey>), "hey")
+#test(str(label("hey")), "hey")
+#test(str([Hmm<hey>].label), "hey")