HCL2 locals don't have type constraints, and the map syntax is interpreted as
an object. So users may need to explictly convert to map for some functions.
The `convert` function documentation was missing the required type constructor
parameter for collections.
Block labels are not expressions so they can't be interpolated without hacks
like `dynamic` blocks. Clarify this so that we don't confuse users who
shouldn't need to dig into the subtle nuances between expressions and blocks.
