[DOC] Added Docstring for regression forecasting

[kavya-r30]

Reference Issues/PRs

Fixes #2563

What does this implement/fix? Explain your changes.

Added Docstring for regression forecasting

Does your contribution introduce a new dependency? If yes, which one?

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I have added the following labels to this PR based on the title: [ $\color{#F3B9F8}{\textsf{documentation}}$ ].
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[baraline]

Small comment on the exog parameter (applies for all functions) and we will be good to go !

[baraline]

LGTM, I'll let other core devs have a look as i'm not that familliar with the forecasting module yet, but should be good to go !