Fix description of "mul" volume notation

[Rosuav]

Description

Small docs fix - this is not a multimap, it's a simple linear multiplier (the square root of the quadratic scale that would be shown to a user).

Motivation and Context

Clarify documentation

How Has This Been Tested?

Looks fine visually. Docs-only change.

Types of changes

• Documentation change (a change to documentation pages)

Checklist:

• [x] I have read the Contributing Guidelines.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] My code is not on master or a release/* branch.
• [x] The code has been tested.
• [x] I have included updates to all appropriate documentation.