Make doc clearer on getCrossSigningKeyId (#4477)
* Make doc clearer on getCrossSigningKeyId
I was trying to work out why this was being used in a check. It
turns out it only returns the key ID if the private part is stored
locally, which seems very much non-obvious.
* Better doc
* Formatting & clarity
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
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Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
