[Enhancement] Update .func.gii Usage and Descriptions for Atlas & Templates

[bendhouseart]

A more "tackle-able" part of the broader discussion about gifti naming and use started with #2416 and #2449, this issue needs to be fixed before the next release (current is v1.11.1).

Adding text to explain .func.gii usage with respect to templates/atlases/surfaces will close this issue.

[bendhouseart]

@mnoergaard @melanieganz @DrCyPhi tagged in

[melanieganz]

As already mentioned in the general PR - in the current version of the spec, we state the following regarding gifti.

Specifically for PET atlasses on the surface (see PR for example here), one could use func.gii. As far as I can see, it's the only one that really fits from the current list.

But I think it's misleading to say that molecular imaging-derived receptor density values are "functional". Any ideas, alternatives?

[DrCyPhi]

@mnoergaard @melanieganz @bendhouseart If I understand correctly, mimap is expected to become an accepted suffix. Wouldn't mimap.gii address the issue of being constrained to use func.gii for molecular imaging surface data (for the time being)?