correct either documentation or behaviour of having a custom parse function

[RonnyPfannschmidt]

in #118 a behaviour issue was discovered when using a custom parse function

since parse functions do not return None when invoked on a invalid root,
exceptions happen

thanks to @anthrotype for pointing this out and investigating a applicable workaround in anthrotype/booleanOperations@74757f3

[RonnyPfannschmidt]

fixed in 951ad9f