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
Adds a few new lines to note directive to fix a build error in the docs.
Removes an obstacle to building the docs.
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