Clarify group ordering better in 'Defining blocks'

[ganicke]

Need to document how group display ordering "really" works. Currently, it is determined not by the order in groups as one would think but by the occurrence of the namespace members. See https://github.com/microsoft/pxt/issues/7258#issuecomment-724948811.

Document this behavior as is or change how the groups are enumerated when displayed?

Closes #7258

[pelikhan]

@aznhassan did you look at the group logic lately?

[FranklinWhale]

I think this PR may need to be amended: https://github.com/microsoft/pxt/issues/7258#issuecomment-728755161