NEXUS-6176: Describe page reasoning.

Manually backported, due to relocation related to content plugin.

Original PR
https://github.com/sonatype/nexus-oss/pull/282

Commits (redone manually, only for reference)
a1a7f9c8b4cf52684951ab1159afb2aeecc578cc
c63df121ac7160978697a9fa6c17761591c33bd0
c2c88bf217dfc59f47c7021e2c28f4d6965877ab
Merge pull request #307 from sonatype/NEXUS-6176-describe-reasoning-backport27x

[2.7] NEXUS-6176: Describe page reasoning.