docs: document pre_blank and post_blank

src/elements/fixed_width.rs

@@ -6,7 +6,10 @@ use crate::parsers::{blank_lines, take_lines_while};
 #[cfg_attr(test, derive(PartialEq))]
 #[cfg_attr(feature = "ser", derive(serde::Serialize))]
 pub struct FixedWidth<'a> {
+    /// Fxied width value
     pub value: Cow<'a, str>,
+    /// Numbers of blank lines between last fixed width's line and next
+    /// non-blank line or buffer's end
     pub post_blank: usize,
 }