Clarify how filters match
Clarify how filters match

This will solve #1073. Any mention of "simple" strings has been removed
and a few pointers have been added that clarify how Fava handles filters
in specific situations.

Additionally, the help page now contains a reference to Python's Regular
Expression Syntax.

yagebu/fava,beancount/fava,beancount/fava,beancount/fava,beancount/fava,aumayr/beancount-web,yagebu/fava,yagebu/fava,beancount/fava,aumayr/beancount-web,aumayr/beancount-web,yagebu/fava,aumayr/beancount-web,yagebu/fava