Update iOS docs to include new GN signing variable.

Also adds some additional detail for tracking down the provisioning
profile's name.

Change-Id: I3ff97bc4b3491bc8f40414e35358bcb459e3a115
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/842656
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed