doc: clarify docs for is_match

This makes it a bit clearer that `is_match` does not require the match
to span the entire string, but rather, that a match may occur anywhere
in the string.

Closes #711

src/re_bytes.rs

@@ -119,7 +119,8 @@ impl Regex {
         RegexBuilder::new(re).build()
     }
 
-    /// Returns true if and only if the regex matches the string given.
+    /// Returns true if and only if there is a match for the regex in the
+    /// string given.
     ///
     /// It is recommended to use this method if all you need to do is test
     /// a match, since the underlying matching engine may be able to do less

src/re_unicode.rs

@@ -175,7 +175,8 @@ impl Regex {
         RegexBuilder::new(re).build()
     }
 
-    /// Returns true if and only if the regex matches the string given.
+    /// Returns true if and only if there is a match for the regex in the
+    /// string given.
     ///
     /// It is recommended to use this method if all you need to do is test
     /// a match, since the underlying matching engine may be able to do less