Update docs for custom trees

[rootulp]

Context

#287

Problem

1. The Go docs for ComputeExtendedDataSquare and ImportExtendedDataSquare don't say anything about the requirements of treeName. I think they should be updated to state that treeName must be registered prior to use.
2. The Go doc for RegisterTree is misleading because RegisterTree can be called by downstream dependencies at any time. There is no hard requirement that RegisterTree must be invoked in an init() function.

rsmt2d/tree.go

Lines 28 to 29 in 5a03c15

	// RegisterTree must be called in the init function
	func RegisterTree(treeName string, treeConstructor TreeConstructorFn) error {

Proposal

Update the Go docs. Ideally before the v0.12.0 release.

[rootulp]

We don't need to do this if we revert the changes. See #295