Remove unused Skirmisher special note from the Orcish Nightblade's description

Found by wmllint. At least as far as I can tell, it's not really
supposed to be a skirmisher unit. Probably. Maybe.
Silence wmllint warnings about an ability-independent special notes macro
NR S1: Add spelling exception for 'slavemaster'
Some spelling/grammar fixes for the editor help text

Still not wmllint-clean, mostly because of help markup issues since
there is no wmllint-visible parent [help] tag to skip spellchecking.

(See also bug #22780.)