Add miscellaneous documentation to some classes

[Mickeon]

This PR adds descriptions for:

• EditorExportPlatformPC itself;
• EditorExportPlatformWindows itself;
• EditorExportPlatformAndroid's xr_features/xr_mode.
• The JSONRPC constants;

I originally wanted to keep this private until I had something more complete, but it's better to create a PR than accidentally losing a bit of work.
And besides, it seems like a bad description is better than no description at all.

[akien-mga]

Thanks!