Documenting when different regexp libraries are used

The documentation hasn't been clear on which regular expression
implementations were used when e.g. interpreting different
configuration variables. In trivial cases this doesn't matter
as most regular expression metacharacters are widely supported,
but for complex expressions it can be beneficial to know.

Change-Id: I2a479aba68779cf857dac9999bc4a90ac2685838

Documentation/access-control.txt

diff --git a/Documentation/access-control.txt b/Documentation/access-control.txt
index 20fe52d..7f5cd44 100644
--- a/Documentation/access-control.txt
+++ b/Documentation/access-control.txt
@@ -155,6 +155,10 @@
 `\^refs/heads/[a-z]\{1,8\}` matches all lower case branch names
 between 1 and 8 characters long.  Within a regular expression `.`
 is a wildcard matching any character, but may be escaped as `\.`.
+The link:http://www.brics.dk/automaton/[dk.brics.automaton library]
+is used for evaluation of regular expression access control
+rules. See the library documentation for details on this
+particular regular expression flavor.
 
 References can have the current user name automatically included,
 creating dynamic access controls that change to match the currently