ApplicationOverride results in "Unable to locate an encrypted key"

[Cantor, Scott]

> It would be _super_ helpful if
> https://wiki.shibboleth.net/confluence/display/SHIB2/NativeSPApplicationO
> verride had a note around 'Mapping via Apache Commands ...' that said you
> need to ensure that /Shibboleth.sso is required to be covered.

I added something, but in future, this is a wiki, if you have improvements, you can just make them directly.

-- Scott