[docs] fix html page for pmc

PackfileSegment PMC is Abstract
The Core PMCs are not Abstract PMC, wrong title
[docs] Fix pmc NAME

Harmonize public PMC names in pod. Fixes GH #1079.
Also do a little bit of PMC description tuning.
[docs] Fix dynpmc NAME

Fixes GH #1079.
ChangeLog: add PMC html index [GH #1079]
Merge branch 'rurban/pmc-podnames-gh1079'

Tested ok