Rollup merge of #41500 - steveklabnik:gh37866, r=frewsxcv

use the word 'length' in Vec::len's docs

Fixes #37866

src/libcollections/vec.rs

@@ -1147,7 +1147,8 @@ pub fn clear(&mut self) {
         self.truncate(0)
     }
 
-    /// Returns the number of elements in the vector.
+    /// Returns the number of elements in the vector, also referred to
+    /// as its 'length'.
     ///
     /// # Examples
     ///