Update docs

Author: Seldaek

README.md

@@ -95,7 +95,8 @@ if (Preg::matchStrictGroups('{fo+}', $string, $matches))
 if (Preg::matchAllStrictGroups('{fo+}', $string, $matches))
 ```
 
-**Note:** This is generally safe to use as long as you do not have optional subpatterns (i.e. `(something)?` or `(something)*`).
+**Note:** This is generally safe to use as long as you do not have optional subpatterns (i.e. `(something)?`
+or `(something)*` or branches with a `|` that result in some groups not being matched at all).
 A subpattern that can match an empty string like `(.*)` is **not** optional, it will be present as an
 empty string in the matches. A non-matching subpattern, even if optional like `(?:foo)?` will anyway not be present in
 matches so it is also not a problem to use these with `*StrictGroups` methods.