Update stage object descriptions, release tags and planning process description

[JachymHercher]

Closes #1385

Closes #866

[jpmckinney]

Just one suggestion. @yolile, feel free to merge if the suggestion is applied.

[jpmckinney]

Regarding:

Planning processes must not use the 'tender' code, even if they use tender fields.

I think the use of "must" here is okay (typically, new "must" statements are not possible in a minor release), because a planning process is identified by the release tag or the tender.status (both of which are expected to align), such that the use of the 'tender' code would imply that the planning process is actually a contracting process – meaning that this new rule can't really be broken. That said, it's still useful to express as a rule, to avoid the blurring of the types of processes.

[JachymHercher]

Just one suggestion. @yolile, feel free to merge if the suggestion is applied.

Just to be sure - did you really mean @yolile or me? :)

[jpmckinney]

I meant @yolile, so that a reviewer merges, rather than the author :) But I can merge now.