BUILD/MEDIUM: da Adding pcre2 support.

The DeviceAtlas Detection API now supports also the pcre2 library,
 and some users wish to have exclusively this version in their
environment.
Also, there is no longer new development happening in the legacy
 pcre(1) counterpart.
Simple check in the build process as the mutual exclusivity check between the
 two are already taking care of early on. Moving the check to the part
only when we build haproxy + the API from source as the other case the API is
 already built with the chosen regex library separately.
diff --git a/doc/DeviceAtlas-device-detection.txt b/doc/DeviceAtlas-device-detection.txt
index 2358c5c..ccafaa0 100644
--- a/doc/DeviceAtlas-device-detection.txt
+++ b/doc/DeviceAtlas-device-detection.txt
@@ -2,10 +2,10 @@
 ----------------------------
 
 In order to add DeviceAtlas Device Detection support, you would need to download
-the API source code from https://deviceatlas.com/deviceatlas-haproxy-module and
-once extracted :
+the API source code from https://deviceatlas.com/deviceatlas-haproxy-module.
+The build supports the USE_PCRE and USE_PCRE2 options. Once extracted :
 
-    $ make TARGET=<target> USE_PCRE=1 USE_DEVICEATLAS=1 DEVICEATLAS_SRC=<path to the API root folder>
+    $ make TARGET=<target> USE_PCRE (or USE_PCRE2=1) USE_DEVICEATLAS=1 DEVICEATLAS_SRC=<path to the API root folder>
 
 Optionally DEVICEATLAS_INC and DEVICEATLAS_LIB may be set to override the path
 to the include files and libraries respectively if they're not in the source