doc: add more explanation to 'CompiledTooBig' error

The existing docs were pretty paltry, and it turns out we can be a bit
more helpful for folks when they hit this error.

Fixes #846

Author: BurntSushi

src/error.rs

@@ -6,8 +6,26 @@ use std::iter::repeat;
 pub enum Error {
     /// A syntax error.
     Syntax(String),
-    /// The compiled program exceeded the set size limit.
-    /// The argument is the size limit imposed.
+    /// The compiled program exceeded the set size
+    /// limit. The argument is the size limit imposed by
+    /// [`RegexBuilder::size_limit`](crate::RegexBuilder::size_limit). Even
+    /// when not configured explicitly, it defaults to a reasonable limit.
+    ///
+    /// If you're getting this error, it occurred because your regex has been
+    /// compiled to an intermediate state that is too big. It is important to
+    /// note that exceeding this limit does _not_ mean the regex is too big to
+    /// _work_, but rather, the regex is big enough that it may wind up being
+    /// surprisingly slow when used in a search. In other words, this error is
+    /// meant to be a practical heuristic for avoiding a performance footgun,
+    /// and especially so for the case where the regex pattern is coming from
+    /// an untrusted source.
+    ///
+    /// There are generally two ways to move forward if you hit this error.
+    /// The first is to find some way to use a smaller regex. The second is to
+    /// increase the size limit via `RegexBuilder::size_limit`. However, if
+    /// your regex pattern is not from a trusted source, then neither of these
+    /// approaches may be appropriate. Instead, you'll have to determine just
+    /// how big of a regex you want to allow.
     CompiledTooBig(usize),
     /// Hints that destructuring should not be exhaustive.
     ///