doc: xss_clean() method is in Security, not Input

The Input doc does talk about xss filtering, but they refer you to Security for details, which is where the function is actually defined.  It gives more detail about what the function is supposed to do, and avoids some irrelevance.

It's probably not a big deal; it just looks wrong.  It _might_ have been responsible for [confusion](http://stackoverflow.com/questions/13570522/this-input-xss-cleandata-giving-fatal-error-with-codeigniter) sometimes.
doc: fix table markup in previous commit

"These "border lines" need to have a length higher or equal to the
longest line in the table that would be generated, so you'll have
to update them as well." - narfbg
Merge pull request #2075 from sourcejedi/patch-1

doc: xss_clean() method is in Security, not Input