docs: explain string paths in get_in

[guhou-hvi]

Addresses #548.

get_in treats its first argument as a sequence of successive keys. A caller
may try get_in('x', {'x': 5}), see it work, and expect a longer string to name
one key too. Instead, get_in('name', {'name': 'Alice'}) returns the default,
and a string like 'xy' can silently select data['x']['y'] instead of
data['xy'].

Explain this in the function's API documentation and show both outcomes next
to the single-key list spelling. The examples use explicit return values,
including a dictionary where the nested path and whole string key differ.
They execute with the project's existing doctest command. This is documentation
of the current behavior; the function body and public API are unchanged.

Validation: the existing pytest/doctest entry reports 268 passed and 2 skipped,
the benchmark suite reports 19 passed, and pycodestyle and strict HTML building
pass. Five new doctest steps live within the existing function's doctest item;
they do not add five pytest items. Independent review checked both direct and
curried usage and confirmed that only the docstring changes.

Codex drafted and tested the documentation; a separate Codex agent reviewed it.
I have reviewed and understand the changes.