Unify terms regarding assembly form to use generic vs. custom

This CL just changes various docs and comments to use the term "generic" and
"custom" when mentioning assembly forms. To be consist, several methods are
also renamed:

* FunctionParser::parseVerboseOperation() -> parseGenericOperation()
* ModuleState::hasShorthandForm() -> hasCustomForm()
* OpAsmPrinter::printDefaultOp() -> printGenericOp()

PiperOrigin-RevId: 230568819