Updated Api changes between v2.1 and v2.2 (markdown)

Api-changes-between-v2.1-and-v2.2.md

@@ -5,6 +5,7 @@
  - Removed the `bind` option for datasources (#1654)
 
 ## Changed
+ - The `max_size` and `initial_size` options for the `PostGIS` datasource impact behavior for a single, global pool of postgresql connections. In Mapnik >= 2.2.0, when set for any layer, if the `max_size` or `initial_size` is larger than the default (10 for `max_size` and 1 for `initial_size`) or any previously set value then the global value will be increased. In older Mapnik only the values set in the first layer would be respected. [Ticket](https://github.com/mapnik/mapnik/issues/1599)
  - Proj4 init errors due to invalid `srs` values will now cause exceptions when a map is loaded from XML as well as when encountered during rendering. The latter will very unlikely happen now that validation is done at map loading time. Previously errors were not reported except at render time and only as warnings printed to stderr (#646)
  - <strike>`<Filter>[attr] != ''</Filter>` now matches only empty strings. This syntax previously matched both empty strings and nulls but this behavior was unintended and buggy given that Mapnik has supported a `null` type since 2.0.0. If you wish to filter out all `attr` values that are not `empty strings` or `null` or `false boolean type` then you can do `<Filter>[attr]</Filter>` or `<Filter>[attr] != '' and [attr] != null and [attr] != false</Filter>`</strike> No longer the case for 2.2 release, due to backwards compatibility fix as per https://github.com/mapnik/mapnik/issues/1859