README.md: Fix indentation
README,md: Rewrite command descriptions

The goals of this change are:

  * to make the command descriptions as clear as possible
  * to make them agree terminologically
  * to make them agree stylistically
  * to bring them on the same level of detail
  * to sync them with code

All the descriptions were written from scratch. After that, they were
cross-checked with the code and the old descriptions to make sure
nothing important was missed.
Merge pull request #390 from yast/readme-command-description-rewrite

[Review] Request from '' @ 'yast/ycp-killer/readme-command-description-rewrite'