docs: update JDK installation notes for macOS (#4442)
docs: update JDK installation notes for macOS (#4442)

* docs: update JDK installation notes for macOS

Install OpenJDK via Homebrew, and document the need to set JAVA_HOME.
Older versions of macOS could also get a "system" Java, but this should work
for any reasonably up-to-date macOS.

* Fix spelling.
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