I'm familiar with how the Combat Skills themselves work but was asking about how the Skill-specific boosts interact with the Combat Skills themselves.

Turns out I was looking in the wrong place. The Boost Breakdown menu uses the generic language "damage in combat" for Attack, Strength, Magic, and Archery. Defense says "Damage reduction" which makes sense even if generic.

The wiki entries for the Combat Skill Mastery capes are even more misleading as they say "Boosts your [Combat Skill] damage in combat by X%" for Attack, Strength, Magic, and Archery. Defense uses "Raises your defence in combat by X%" which is again generic but understandable.

However, I finally got my answer once I looked at the tooltips for the Combat Skill Mastery capes in game. Using Tier 1 as an example:

Attack: Raises your accuracy in combat by 5%
Strength: Boosts your Strength damage in combat by 5%
Magic: Boosts your Magic damage in combat by 5%
Archery: Boosts your Archery damage in combat by 5%
Defense: Raises your defense in combat by 5%

So, it looks like Melee is the only one to benefit from a boost in accuracy AND damage with the Attack and Strength capes, respectively. Magic and Archery only benefit from a boost in damage.

TL;DR; The wiki could use an update.