Add documentation for `len` field on `Storage`

Because the purpose of the `len` field wasn't obvious and collided with
other uses (like Vec::len()), some additional documentation has added to
make things a little easier to understand.

src/grid/storage.rs

@@ -20,6 +20,13 @@ pub struct Storage<T> {
     inner: Vec<T>,
     zero: usize,
     visible_lines: Line,
+
+    /// Total number of lines currently active in the terminal (scrollback + visible)
+    ///
+    /// Shrinking this length allows reducing the number of lines in the scrollback buffer without
+    /// having to truncate the raw `inner` buffer.
+    /// As long as `len` is bigger than `inner`, it is also possible to grow the scrollback buffer
+    /// without any additional insertions.
     #[serde(skip)]
     len: usize,
 }