Improved pillar docs

This commit adds documentation on the pillar.get function, and also
converts a couple references to some of the execution functions to links
to the docstring for that function, for better navigation.
Add information on new features

I've recently added these two features and figured they deserve a
mention in the release notes.
Slight grammar fix

Removed an unneeded "the".
Merge pull request #5619 from terminalmage/release_notes

Add information on new features
Merge pull request #5618 from terminalmage/docs

Improved pillar docs