Update custom.js

Explain different semantics required for instances versus functions or classes. Based on comments of @minrk in #5985. Makes it clear not everything needs to be stuck in an event monitor or external script. 
Warning, if `IPython.Cell.options_default.cm_config.extraKeys` already have extra keys defined (hit, it has), this will remove the already assigned `extraKeys`. For example, 'up' and 'down' are bound to 'visual line up' and 'visual line down' 

IMHO, it would be better to do  
```
IPython.Cell.options_default.cm_config.extraKeys['Home'] = 'goLineLeft';
IPython.Cell.options_default.cm_config.extraKeys['End'] = 'goLineRight';
```
Does it makes sens ?