Fix docstring in alternatives module

This makes the autosummary in http://docs.saltstack.com/ref/modules/all/
have a proper summary text.
Fix docstring in eselect module

This also makes the autosummary used in
http://docs.saltstack.com/ref/modules/all/ look better.
Add top docstring to portage_config module

This makes the autosummary on
http://docs.saltstack.com/ref/modules/all/index.html look better.
Fix more top-level docsrings to improve docs
Merge pull request #7261 from terminalmage/issue7250

Fix docstring in alternatives module