Add table of built-in cosmologies, other minor documentation cleanups.

* closes #1147
Fix sphinx warning, add note about doc update

* Extra space before code block in Built-in Cosmologies section
  of index.rst
* Add warning to parameters.py to update the documentation table
  when adding a new cosmology
Merge pull request #1527 from aconley/cosmo_builtin_docs

Add table of built-in cosmologies, other minor documentation cleanups.