doc: tweak docs for 'shortest_match'

The name is somewhat unfortunate, but it's actually kind of difficult to
capture the right semantics in the name. The key bit is that the
function returns the offset at the point at which a match is known, and
that point might vary depending on which internal regex engine was used.

Fixes #747

src/re_bytes.rs

@@ -549,7 +549,14 @@ impl Regex {
     /// This method may have the same performance characteristics as
     /// `is_match`, except it provides an end location for a match. In
     /// particular, the location returned *may be shorter* than the proper end
-    /// of the leftmost-first match.
+    /// of the leftmost-first match that you would find via `Regex::find`.
+    ///
+    /// Note that it is not guaranteed that this routine finds the shortest or
+    /// "earliest" possible match. Instead, the main idea of this API is that
+    /// it returns the offset at the point at which the internal regex engine
+    /// has determined that a match has occurred. This may vary depending on
+    /// which internal regex engine is used, and thus, the offset itself may
+    /// change.
     ///
     /// # Example
     ///

src/re_unicode.rs

@@ -607,7 +607,14 @@ impl Regex {
     /// This method may have the same performance characteristics as
     /// `is_match`, except it provides an end location for a match. In
     /// particular, the location returned *may be shorter* than the proper end
-    /// of the leftmost-first match.
+    /// of the leftmost-first match that you would find via `Regex::find`.
+    ///
+    /// Note that it is not guaranteed that this routine finds the shortest or
+    /// "earliest" possible match. Instead, the main idea of this API is that
+    /// it returns the offset at the point at which the internal regex engine
+    /// has determined that a match has occurred. This may vary depending on
+    /// which internal regex engine is used, and thus, the offset itself may
+    /// change.
     ///
     /// # Example
     ///
@@ -627,12 +634,12 @@ impl Regex {
         self.shortest_match_at(text, 0)
     }
 
-    /// Returns the same as shortest_match, but starts the search at the given
-    /// offset.
+    /// Returns the same as `shortest_match`, but starts the search at the
+    /// given offset.
     ///
     /// The significance of the starting point is that it takes the surrounding
-    /// context into consideration. For example, the `\A` anchor can only
-    /// match when `start == 0`.
+    /// context into consideration. For example, the `\A` anchor can only match
+    /// when `start == 0`.
     pub fn shortest_match_at(
         &self,
         text: &str,