docs/build-run: Add further context on Xcode 11 / old-RN issue.
docs/build-run: Add further context on Xcode 11 / old-RN issue.

Also link directly to the commit that was ultimately merged.
(Found from the PR thread; it's linked at the spot where the
PR gets closed.)  That commit has the PR's description, and
also links to the PR in case the reader is interested in the
further information there.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile