Blender Git Loki

Docs: Add to and cleanup attribute API docs

Most of the comment block is similar to the text in the source
code documentation wiki. It's helpful to have some text in
a header file too, so it's closer for programmers already looking
at the code.

This also uses more consistent syntax and wording in the comments
about the attribute API in `GeometryComponent`.

Ref T93753

Differential Revision: https://developer.blender.org/D13661