Merge pull request #2837 from iBotPeaches/feature/user-guide-cleanup

fix outdated php.net url
Add &newline; and &tab; to CI_Security::
CI_Security: Add 'form' and 'xlink:href' to evil attributes
Fix syntax errors
CI_Security: Add <select> and <keygen> tags to the list of 'naughty' HTML elements
CI_Security: Also add <svg> to 'naughty' HTML elements
[ci skip] Add a link to PHP bug 54709 in is_really_writable()'s docblock
Merge branch 'develop' into feature/user-guide-cleanup
[ci skip] Add a note to the is_really_writable() docs
fix warning