Add support for Boost.Ref in match_results::format.

Update docs accordingly.
Fixes #4020.

[SVN r60678]

libs/regex

@@ -1 +1 @@
-Subproject commit 446604c734667246da40e0694d75b34311847707
+Subproject commit 67f5369cae0fcd02b39779a1484f2c4f56239a50