Allow {{faq}} macro to display entries from different FAQs

The filter parameter of the faq macro also gives no message when its value is wrong: an unknown key like bogus=1, an unknown space like space=NoSuchSpace, a wrong case like space=faq, a key without a value, spaces around the equal sign or plain text all give the same empty table, "Results 0 - 0 out of 0". Since this issue reworks the same parameter, the new behavior could also report a filter it cannot use, for example an unknown key or a space with no FAQ, instead of an empty table. Declaring the format of the value is covered by XWIKI-25197.

This KO comes from the WYSIWYG usability audit of the macros, under the criterion E2 errors of its usability criteria.