www/firefox: document missing feature frequently asked on IRC/BMO

about:crashes is not supported on most Tier3 platforms. Even if someone
adopts devel/breakpad patches while filling the gaps without reproducibility
debugging symbols uploaded to Mozilla may not match the package.

https://searchfox.org/mozilla-central/rev/403038737ba7/old-configure.in#2699-2718
https://developer.mozilla.org/docs/Mozilla/Developer_guide/Uploading_symbols_to_Mozillas_symbol_server