obs-webrtc: Various WHIP patches

[tt2468]

Description

• Fixes ifdef guards for HEVC
• Fixes a capitalization of a few comments
• Only advertises relevant codecs in audio-only and video-only outputs

Motivation and Context

Clean things up and improve standards

How Has This Been Tested?

OBS compiles

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.