Explain non-deterministic function cases in docs

[kathia-barahona]

Issue: Some functions in both DataFrame and Series might yield different results when sorting is involved.

Proposal: dedicate a section in the docs where this issue is well explained and clear for users.

Background: https://github.com/objectiv/objectiv-analytics/pull/492#discussion_r811820706_.

[thijs-obj]

Functions that can be potentially (depending on data and paramters) non-deterministic:

• fillna()
• drop_duplicates()
• window_row_number()
• window_rank()
• mode()