PBAP: databaseFormat + filter fields

Print filter fields as part of the debug output and document the
semantic of databaseFormat (was only in a commit message before).

src/backends/pbap/PbapSyncSource.cpp

@@ -255,6 +255,8 @@ void PbapSession::initSession(const std::string &address, const std::string &for
     // get filter list so that we can continue validating our format specifier
     std::vector<std::string> filterFields =
         GDBusCXX::DBusClientCall1< std::vector<std::string> >(*m_session, "ListFilterFields")();
+    SE_LOG_DEBUG(NULL, NULL, "supported PBAP filter fields:\n    %s",
+                 boost::join(filterFields, "\n    ").c_str());
 
     std::list<std::string> filter;
     if (negated) {

src/backends/pbap/README

@@ -9,6 +9,50 @@ be running.
 The "database" property specifies the Bluetooth address of the
 device to be connected.
 
+The "databaseFormat" can be used as follows:
+
+      [(2.1|3.0)][:][^]propname,propname,...
+
+      3.0 = download in vCard 3.0 instead of the default 2.1
+      3.0:^PHOTO = download in vCard 3.0 format, excluding PHOTO
+      PHOTO = download in vCard 2.1 format, only the PHOTO
+
+Valid property names are the ones listed by obexd in
+ListFilterFields(). These fields may include:
+     VERSION
+     FN
+     N
+     PHOTO
+     BDAY
+     ADR
+     LABEL
+     TEL
+     EMAIL
+     MAILER
+     TZ
+     GEO
+     TITLE
+     ROLE
+     LOGO
+     AGENT
+     ORG
+     NOTE
+     REV
+     SOUND
+     URL
+     UID
+     KEY
+     NICKNAME
+     CATEGORIES
+     CLASS
+     BITxy (for different values of xy)
+
+Set "SYNCEVOLUTION_DEBUG=1" as environment variable when using the
+backend to see a list of filter fields supported by obexd.
+
+Syncing
+=======
+
 The backend itself is not able to detect changes since the last
 sync, mostly because PBAP as a protocol makes that hard. For example,
 it makes no guarantee that an item has the same ID in two different