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Fix some miscellaneous typos in API docs.

See merge request !256

make sure to initialize the 4th entry in MVertex* vector, so that the hash value is deterministic for triangular faces

Julia API, NFC: Use Cvoid instead of Nothing in ccalls

See merge request !255

and for return types in write_function. Cvoid is just an
alias for Nothing, but recommended to used in ccalls
for readability.