added wiki comments + wiki-generating python script

src/precedence.rs

@@ -5,6 +5,17 @@ use syntax::ast_util::binop_to_string;
 
 use utils::{span_lint, snippet};
 
+/// **What it does:** This lint checks for operations where precedence may be unclear and `Warn`'s about them by default, suggesting to add parentheses. Currently it catches the following:
+/// * mixed usage of arithmetic and bit shifting/combining operators without parentheses
+/// * a "negative" numeric literal (which is really a unary `-` followed by a numeric literal) followed by a method call
+///
+/// **Why is this bad?** Because not everyone knows the precedence of those operators by heart, so expressions like these may trip others trying to reason about the code.
+///
+/// **Known problems:** None
+///
+/// **Examples:**
+/// * `1 << 2 + 3` equals 32, while `(1 << 2) + 3` equals 7
+/// * `-1i32.abs()` equals -1, while `(-1i32).abs()` equals 1
 declare_lint!(pub PRECEDENCE, Warn,
               "catches operations where precedence may be unclear. See the wiki for a \
                list of cases caught");