Docs - Update CSW proxy weapon information

[johnb432]

When merged this pull request will:

• Explicitly say that a proxy weapon is optional.
• Document that proxy weapon entry can be a function name. In ACE, it's only used in the mortar framework, but that will be removed with CSW/Mk6 Mortar - Make Mk6 Mortar Ammo Handling override CSW's #9238. Question is: Do we really want to add it as API or leave it undocumented?

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