Logging functions: level parameter is not optional

This parameter cannot be optional, as the following parameter is mandatory.

Also completed the corresponding documentation.
Fix a docblock in Loader class
Do not trigger a possible custom autoloader, as it is irrelevant here

These were the last two calls of class_exists() without the $autoloader = FALSE argument.
Merge pull request #2440 from vlakoff/develop

Logging functions: level parameter is not optional
Merge pull request #2441 from vlakoff/develop-2

Fix a docblock in Loader class
Merge pull request #2442 from vlakoff/develop-3

Do not trigger a possible custom autoloader, as it is irrelevant here