manual: Various fixes and improvements

manual/src/builtins.md

manual/src/control/01-conditionals.md

manual/src/control/03-matches.md

manual/src/expansions/02-process.md

manual/src/expansions/04-arithmetic.md

manual/src/expansions/05-method.md

manual/src/functions.md

manual/src/history.md

manual/src/scripts.md

manual/src/slicing.md