Fixed a typo in a code comment for LM-Response
A number of fixes (mainly comments) of rlm_expr

* Update the urlunquote example to make it copy-pastable by doubling the %. Add a comment why
* Typo fix: s/wont'/won't/
* Removed UNUSED macro for *instance in (un)escape_xlat, it is being used here
* Fix the unuescape example, the name here should be unescape, not escape
* Added a ":" after "Example" in tolower. This makes it easier to find the examples in this file, since all other examples did this.