The first page is in the documentation for MATLAB Coder and discusses how to generate C/C++ code from MATLAB code in general, listing the various options that are available. The examples on that page are small and assume you have already set up your system to generate code.
The second page is in the documentation for Image Processing Toolbox and shows how to generate code from a function that uses Image Processing Toolbox functionality. It shows one example that goes through the whole workflow, from setup to the final generated code.
The documentation for the watershed function does indicate it is supported for code generation, but it also lists some usage notes and limitations at the end of the page.