DOC: Remove internally used/private parameters from method signatures

[NumanIjaz]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.Series.html#pandas.Series

Documentation problem

The public api contains internally used parameter. In this case the Series constructor contains fastpath parameter which isn't documented in the docstring. However, is part of the constructor signature.

Suggested fix for documentation

Suggested solution:

1. Prefix the internally used/private parameter with an underscore (_)
2. Remove all the parameters starging with an underscore while processing the method signatures in sphinx.

This problem is already identified in a very old issue: #6903

[phofl]

@DeaMariaLeon why did you remove the needs discussion label?

[DeaMariaLeon]

@phofl I read "needs triage" (and it is still shown on the upper right area like that).. sorry

[topper-123]

We have an issue open about removing the private parameters (fastpath etc.) from the public interfaces, see #20110. Getting that done would be the proper solution to this problem, so I'm probably -1 on doing special preprocessing in sphinx etc. and IMO this specific doc issue be closed.

However, getting fastpath deprecated (and its functionality added in an internal class method) would be very nice and PRs in that direction would be welcome.