Improve documentation

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

doc/configuration.rst

@@ -531,6 +531,14 @@ Options
         * With ``greedy_method = "build" ``pkg-b-provider-1`` and
           ``pkg-b-provider-2`` will be pulled in.
 
+**dnf_gather** = False
+    (*bool*) -- When set to ``True``, DNF backend is used instead of YUM. This
+    changes the entire codebase doing dependency solving, so it can change the
+    result in unpredictable ways.
+
+    Particularly the multilib testing is performed differently with much less
+    magic. Please refer to ``multilib`` option to see the differences.
+
 
 **multilib_methods** [deprecated]
     ([*str*]) -- use ``multilib`` instead to configure this per-variant
@@ -545,11 +553,19 @@ Options
     methods
 
     Available methods are:
-     * ``none``
-     * ``all``
-     * ``runtime``
-     * ``kernel``
-     * ``yaboot``
+     * ``none`` -- no package matches this method
+     * ``all`` -- all packages match this method
+     * ``runtime`` -- packages that provide something matching
+       ``*.so.[0-9]+.*`` will match. With ``yum`` backend installed files are
+       also looked at for a match with a hardcoded list of patterns.
+     * ``devel`` -- packages whose name ends with ``-devel`` or ``--static``
+       suffix will be matched or packages that provide something with such
+       suffix. With ``yum`` backend this method also uses a hardcoded blacklist
+       and whitelist.
+     * ``kernel`` -- packages providing ``kernel`` or ``kernel-devel`` match
+       this method (only in ``yum`` backend)
+     * ``yaboot`` -- only ``yaboot`` package on ``ppc`` arch matches this (only
+       in ``yum`` backend)
 
 **additional_packages**
     (*list*) -- additional packages to be included in a variant and