Improve collection modifying API docs

Load collection:
- state explicitly that 'count' attribute needs to be in request body
object
- add 'isSystem' attribute to result description

Unload collection:
- add 'isSystem' attribute to result description

Change collection properties:
- modify return description
- add 'isSystem', 'isVolatile', 'doCompact' and 'keyOptions' attributes
to result description

Rename collection:
- add 'isSystem' attribute to result description

Rotate collection journal:
- fix return code typo