Documentation improvements

Codeigniter is known for its good documentation, but that doesn't mean it can't be improved. I'd like to suggest a Documentation subforum under Issues, where we can point out errors and suggest improvements. I think this is important, especially considering that many people starting with Codeigniter are also new to PHP.

Here is a concrete example. The documentation states under the section Callbacks: Your own Validation Functions,

To invoke a callback just put the function name in a rule, with "callback_" as the rule prefix. If you need to receive an extra parameter in your callback function, just add it normally after the function name between square brackets, as in: "callback_foo[bar]", then it will be passed as the second argument of your callback function.

The above is not incorrect but it can lead novice programmers to tear their hair out. The problem is that the example shows how to pass the string 'bar' to the callback function, yet the more common case would be passing the value in a variable. And to pass the value of variable $bar, the example should read "callback_foo[$bar]" while reminding the reader that double-quotes must be used if PHP is to parse $bar as a variable. The documentation could further clarify,

To pass the value of a variable, you must use concatenation, double quotes, or complex syntax. Like this:
concatenation: 'callback_foo['. $bar . ']'
double quotes: "callback_foo[$bar]"
complex syntax: "callback_foo[{$bar['snafu']}]"
That last style is especially helpful when passing array elements.

The documentation could also link to the PHP manual, specifically to:
Strings - Variable Parsing, and Why is $foo[bar] wrong?.

You can quibble with the example I'm giving, but I still think we can make a community effort to improve the manual. What do you think?