Add docs to blocks context postprocessing function

[Ian-GL]

Description

There is an issue adding 'api_name' to a function that updates a column. The stacktrace showed this was the error:

File "/Users/ian/Development/gradio/gradio/documentation.py", line 45, in document_fn
    doc_lines = doc_str.split("\n")
AttributeError: 'NoneType' object has no attribute 'split'

The function that the doc_str referred to in this case was BlockContext.postprocess, so just adding docs to that function was enough to fix the issue.

Closes: #2251

Checklist:

• I have performed a self-review of my own code
• My code follows the style guidelines of this project
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[freddyaboulton]

Thanks @Ian-GL !