Fix #12541: mc_issue_note_add function not honoring reporter data

http://www.mantisbt.org/bugs/view.php?id=12541

Applied the patch in the bug with some changes, credited to the contributor.  Then added another one to add documentation to the manual.

The change doesn't break existing test cases, but it is not possible to add a test case for the new functionality right now.  The reason is that it is not possible to add a secondary user from SOAP API right now due to lack of such APIs.  I believe we have patches for adding such functionality that I will try to look into.

The new functionality is pretty much a copy and paste from adding a new issue with a reporter other than the signed in user.
Looks like we have an unnecessary assignment and test - suggest to simplify the code

``` php
if( isset( $p_note['reporter'] ) ) {
	$t_reporter_id = mci_get_user_id( $p_note['reporter'] );
	if( $t_user_id != $t_reporter_id ) {
		[...]
	}
} else {
	$t_reporter_id = $t_user_id
}
```