added wiki comments + wiki-generating python script

src/len_zero.rs

@@ -11,10 +11,29 @@ use syntax::ast::Lit;
 
 use utils::{get_item_name, snippet, span_lint, walk_ptrs_ty};
 
+/// **What it does:** This lint checks for getting the length of something via `.len()` just to compare to zero, and suggests using `.is_empty()` where applicable. It is `Warn` by default.
+///
+/// **Why is this bad?** Some structures can answer `.is_empty()` much faster than calculating their length. So it is good to get into the habit of using `.is_empty()`, and having it is cheap. Besides, it makes the intent clearer than a comparison.
+///
+/// **Known problems:** None
+///
+/// **Example:** `if x.len() == 0 { .. }`
 declare_lint!(pub LEN_ZERO, Warn,
               "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` \
                could be used instead");
 
+/// **What it does:** This lint checks for items that implement `.len()` but not `.is_empty()`. It is `Warn` by default.
+///
+/// **Why is this bad?** It is good custom to have both methods, because for some data structures, asking about the length will be a costly operation, whereas `.is_empty()` can usually answer in constant time. Also it used to lead to false positives on the [`len_zero`](#len_zero) lint – currently that lint will ignore such entities.
+///
+/// **Known problems:** None
+///
+/// **Example:**
+/// ```
+/// impl X {
+///     fn len(&self) -> usize { .. }
+/// }
+/// ```
 declare_lint!(pub LEN_WITHOUT_IS_EMPTY, Warn,
               "traits and impls that have `.len()` but not `.is_empty()`");