Improve the documentation on get_idp() function

[PeaceNlove]

Describe the bug
On this page the get_idp() function is explained:
https://developers.arcgis.com/python/latest/api-reference/arcgis.gis.admin.html#arcgis.gis.admin.SecuritySettings.get_idp

It it stated that this function works only on AGOL, however with the 2.4.2 release it looks like it also works with Portal. Can you remove the text ArcGIS Online Only.

To me it's unclear that this function only works for SAML and not for OpenID Connect, can you clarify in the documentation that it's only for SAML based identity federation

Thank you

[nanaeaubry]

@tarunkukreja003