[1.6.x] Included irc link in paragraph to allow for l10n

Translators may want to change the irc link pointing to a
language-specific channel.
Backport of 232f2028d5 from master.
[1.6.x] Fixed #13755 -- Added a tip for caching responses that vary on AJAX.

Thanks mila for the suggestion.

Backport of db9a1a0c47 from master