Moved info about the change to function's doc

As discussed with @mwiencek, we don't need to expose those to average user,
nor to mark those for translations.
Shorten lines using intermediate variables
Output hooks docstrings to log.debug

Example of output:
```
D: 10:57:46 Config upgrade 1.3.0dev1 -> 1.3.0dev2: Option "preserved_tags" is now using comma instead of spaces as tag separator (PICARD-536)
```