First pass at improving OLX docs

docs/en_us/olx/source/problem-xml/index.rst

@@ -1,10 +1,23 @@
 .. _Problems and Tools:
 
 #################################
-Problems and Tools
+Problems
 #################################
 
-Use the chapters in this section to create course problems and tools.
+The format for edX problems is based on the LONCAPA XML format (https://s1.lite.msu.edu/adm/help/Authoring_XML_Intro.hlp#Authoring_XML_Intro), although the two are not quite compatible. In the edX variant, problems are composed of four types of tags: 
+
+* inputtypes are similar to XBlocks. They define ways for users to enter input into the problem. 
+* responsetypes are graders. They define how inputs are mapped to grades. 
+* hinters are used to provide feedback to problems. 
+* Standard HTML tags are used for formatting. 
+
+In abstract, the system is designed to allow mixing-and-matching of inputtypes, responsetypes, and hinters. For example, a numerical grader could match 7+-0.1%. It would be okay to use this with any inputtype which output a number, whether this was a text box, equation input, slider, or multiple choice question. In practice, this doesn't always work. For example, in the former case, a multiple choice question would not give an output in a format a numerical grader could handle. 
+
+In addition, in many cases, there is a 1:1 mapping between graders and inputs. For some types of inputs (especially discipline-specific specialized ones), it simply does not make sense to have more than one grader. 
+
+The most general grader is customresponse. This uses a piece of Python code to evaluate the input. By design, this ought to work with any inputtype, although there are bugs mixing this with a small number of the newer inputtypes. 
+
+Like LON-CAPA, OLX allows embedding of code to generate parameterized problems. Unlike LON-CAPA, edX supports Python (and not Perl). Otherwise, the syntax is approximately identical. 
 
 .. toctree::
    :maxdepth: 2
@@ -12,16 +25,11 @@ Use the chapters in this section to create course problems and tools.
    checkbox
    chemical_equation
    circuit_schematic_builder
-   conditional_module
    custom_javascript
    custom_python
    drag_and_drop
    dropdown
-   full_screen_image
    gene_explorer
-   google_hangout
-   graphical_slider_tool
-   iframe
    image_mapped_input
    math_expression_input
    mathjax
@@ -29,12 +37,8 @@ Use the chapters in this section to create course problems and tools.
    mult_choice_num_input
    multiple_choice
    numerical_input
-   periodic_table
-   poll
    problem_in_latex
    problem_with_hint
    protein_builder
    symbolic_response
    text_input
-   vitalsource
-   zooming_image