doc: Improve Javadoc quality in Environment interface

[slarse]

#3923

The Environment interface is arguably one of the most important ones in the entire Spoon library, and it's fairly underdocumented. This PR improves the documentation by adding all missing @param and @return tags, fixing errors and adding missing explanations of what things actually do.

Javadoc method quality score increases like so:

JAVADOC QUALITY SCORE (lower is better)
    Compare: 2641
    Current: 2584

There are still improvements to be made here, especially in explaining what the options you can set actually do, but I felt like this was enough for a single PR.

[monperrus]

We ❤️ documentation PR. Thanks Simon.