doc: update filters documentation

docs/mapping.rst

@@ -92,14 +92,14 @@ Some column types require additional arguments. Refer to the documentation of th
 ``from_member`` is only valid for tables of the type ``relation_member``. If this is set to ``true``, then tags will be used from the member instead of the relation.
 
 
-``filter``
-~~~~~~~~~~
+``filters``
+~~~~~~~~~~~
 
 You can limit which elements should be inserted into a table with filters.
 You can ``require`` specific tags or ``reject`` elements that have specific tags.
 ``require`` and ``reject`` accept keys and a list of values, similar to a ``mapping``. You can use ``__any__`` to require or reject all values (e.g. ``amenity: [__any__]``).
 
-``require_regexp`` and ``reject_regexp`` can be used to filter values based on a regular expression.
+``require_regexp`` and ``reject_regexp`` can be used to filter values based on a regular expression. You can use the `Go Regex Tester <https://regex-golang.appspot.com/assets/html/index.html>`_ to test your regular expressions.
 
 The following mapping only imports buildings with a `name` tag. Buildings with ``building=no`` or ``building=none`` or buildings with a non-numeric level are not imported.
 
@@ -108,7 +108,7 @@ The following mapping only imports buildings with a `name` tag. Buildings with `
     tables:
       buildings:
         type: polygon
-        filter:
+        filters:
           require:
             name: [__any__]
           reject:
@@ -124,6 +124,11 @@ The following mapping only imports buildings with a `name` tag. Buildings with `
 
   Regular expressions in ``require_regexp`` and ``reject_regexp`` should be enclosed in single quotes (``'``). Otherwise YAML will interpret backslashes as escape sequences.
 
+.. note::
+
+  You can only filter tags that are referenced in the ``mapping`` or ``columns`` of any table. See :ref:`tags` on how to make additional tags available for filtering.
+
+
 Example
 ~~~~~~~