Clarify swing_modes allows custom values in ClimateEntity

[gjohansson-ST]

Background

Many integrations sets custom values in swing_modes but it isn't clarified in the developer documentation if this is allowed or not.
There is also no presence in either core code or in previous discussions/issues if this was the intention or not (while it would seem that it would as for every device the possible swing modes would be different).

For both preset and for fan it's clarified in the dev docs that it's allowed for integrations to add their own custom values in addition to using the standard ones.

Proposal

Clarify in the developer documentation that similar to preset and fan, also swing modes allows integrations to append integration/device specific swing modes to the list.

Alternatives

None as it would not make sense to limit only to a standard provided list (as swing modes are device specific and different for most).