Updated the documentation for the fastfn function with default argume…

[Swanand01]

Updated the doctstrings of the fastfn() function in the model.py file to include the missing argument and the default values.
Also replaced **kargs with **kwargs in the model.py file

Thank your for opening a PR!

Depending on what your PR does, here are a few things you might want to address in the description:

• what are the (breaking) changes that this PR makes?
• important background, or details about the implementation
• are the changes—especially new features—covered by tests and docstrings?
• linting/style checks have been run
• consider adding/updating relevant example notebooks
• right before it's ready to merge, mention the PR in the RELEASE-NOTES.md

Closes #4630

[michaelosthege]

Hi @Swanand01 ,
it looks like your PR goes into master and it changes code that, if I'm not mistaken, is heavily refactored on the v4 branch.
Because we want to merge v4 into master, we have frozen PRs into master for the time being. Also see https://github.com/pymc-devs/pymc3/wiki/Timeline about this.
If the problem you want to address still exists on the v4 branch we'll be happy to take a PR there.