Update mapper documenation for match helper [ci skip]

This piece of documentation is out of date.
The use of match without any via option is prevented in rails 4,
the HTTP verbs have to be explicitly set. If they're not set then
the error message in normalize_conditions! (around line 186) is
shown.

(As a side note, this is my first pull request to rails. I think I'm following 
the format for PR's, but if there's something else I should be doing please 
let me know.)