Docs: explain wrappers

[Sjors]

The site currently says: "Fragments that do not change the semantics of their subexpressions are called wrappers." and a bit below it says "t:, l:, and u: wrappers are syntactic sugar for other Miniscripts".

The wrappers with their opcodes are listed under "X (identities)" in the translations table.

Other than the c: and v: wrappers, their purpose is rather mysterious to me.