Clarify dimension doc.

Fix doc creation script.

doc/conf.py

@@ -87,12 +87,15 @@ def read_version(filename):
     # project(PDAL VERSION 0.9.8 LANGUAGES CXX C)
     data = open(filename).readlines()
 
-    token = 'VERSION'
+    token = 'PDAL VERSION'
 
+    version = None
     for line in data:
         if str(token) in line:
-            version = re.search(r'\d.\d.\d', line).group(0)
-            break
+            match = re.search(r'\d.\d.\d', line)
+            if match is not None:
+                version = match.group(0)
+                break
     return version
 
 release = read_version('../CMakeLists.txt')

doc/dimensions.rst

@@ -4,9 +4,17 @@
 Dimensions
 ===============================================================================
 
-PDAL dimensions describe the combination of data's type, size, and meaning. The
-following table provides a list of known dimension names you can use in
-:ref:`filters`, :ref:`writers`, and :ref:`readers` descriptions.
+All point data in PDAL is stored as a set of dimensions.  Dimensions
+have a name and a data type.  The data type is determined at runtime, but
+a default data type for each dimension is listed below, along with the name
+of the dimension and its description.
+
+The following table provides a list of known dimension names you can use in
+:ref:`filters`, :ref:`writers`, and :ref:`readers`.
+
+.. note::
+    Types are default types. Stage developers should set the dimension type
+    explicitly if the default dimension isn't suitable.
 
 
 .. csv-table::