DOC: Update the DataFrame.copy docstring to note different behaviour with Copy-on-Write

[jorisvandenbossche]

Currently the copy() method docstring shows several examples how mutations are propagated for shallow copies ("Any changes to the data of the original will be reflected in the shallow copy (and vice versa)." in the main description, "Updates to the data shared by shallow copy and original is reflected in both; deep copy remains unchanged." in the examples section)

While is this of course still correct with the default behaviour, given that this is completely incorrect when CoW is enabled, I would at least start with adding a note about this (and for example referring to the Copy-on-Write documentation).

[punndcoder28]

Hi! I can take this up if it is not already being worked on.

[sqali]

Hi @jorisvandenbossche ,

I would love to take up this issue. May I have the link to the file that we need to update?