Add docstrings to public methods from context.c

[plamut]

This PR adds docstrings to the functions from context.c. It essentially just copies the relevant bits from the existing documentation on Context Variables.

(some of the text was adjusted a bit to adhere to the 80 characters limit of the docstring summary line)

@1st1 Please just let me know your final decision on whether or not to create a ticket for this, and I'll update the PR title accordingly if needed, thanks.

[the-knights-who-say-ni]

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[plamut]

For the record, I just signed the CLA, and we now need to wait one business day for an official confirmation.