Update Combat help for 2.6
A couple of tweaks to the "Combat" section of helpdata.txt:
- 'killcitizen' server option. (This should have been done in 2.5.)
- Generalisation of 'shore bombardment' firepower reduction (gna patch #3830, commit:c21fcd30f, debuts in 2.6).
Also, add comments in helpdata.txt about where the various effects mentioned come from in the code/ruleset.