Improve wording to emphasize static behavior about measuring maintenance

Currently, `Very Popular` and `Popular` badges has the following descriptions:
- **Very Popular :** The popularity of a project is a useful way of discovering if it is useful, **_and well maintained**_.
- **Popular :** A popular library means there can be a community to help improve and **_maintain**_ a project.

However, we currently do not have such metrics to measure maintenance over time, and the wording does not imply the fact that all these measurements are done according to the state of the pod repo at the release time.

Also some badges like `Lots of open issues` could no more be valid when some time passed after the release, or the reverse.

I think we should improve wording a little, but as a foreign I do not have exact suggestions.