minor #874 Update http_cache.rst - hint at whitelisting 127.0.0.1 if using HTTP Cache solution (alcohol)

This PR was squashed before being merged into the 1.2.x-dev branch (closes #874).

Discussion
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Update http_cache.rst - hint at whitelisting 127.0.0.1 if using HTTP Cache solution

Took me quite a while to realise this; this simple tip might help others save that time :-)

Commits
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4b1ae82b Update http_cache.rst - hint at whitelisting 127.0.0.1 if using HTTP Cache solution