TraitType.make_dynamic_default is not mentioned in the docs

[liamhuber]

Per the title, there is no mention of make_dynamic_default for custom TraitTypes in the docs right now. The current example uses a tuple with two immutable elements, so it's technically safe, but it is at best incomplete (for people who want a mutable default) and at worst misleading (for people who thoughtlessly use a list or other mutable type instead of tuple, or put something mutable inside such a tuple).

The code itself is very nicely written, so once I decided it ought to be possible to safely have mutable defaults for children of TraitTypes I had no trouble finding the search for make_dynamic_default, but this should be better highlighted in the public-facing documentation.