Fix CONSTRAINTS_DOC formatting in modeling docs
Update operations.rst

Fixed typo.
Fixed double backticks
Fixed group_by references
Fixed Table and Column references
Document return value of discretize_model
Fix typo in Model2DKernel docstring.

(It takes a 2D model, not a 1D model as input.)
changed input parameter from "width" to "fwhm" for Gaussian kernels
changed input parameter from "width" to "stddev" for Gaussian kernels
updated docs
Added change log entry for #2085
Merge pull request #2120 from cdeil/issue_2119

Fix CONSTRAINTS_DOC formatting in modeling docs
Merge pull request #2253 from larrybradley/patch-4

Update operations.rst
Merge pull request #2272 from cdeil/kernel_typos

Fix minor typos for discretize_model and Model2DKernel docs
Updated Gaussian2DKernel example in whatsnew/0.3.rst
Merge pull request #2085 from larrybradley/gausskerns

Gaussian kernels: changed input parameter from "width" to "fwhm"