docs(guide/components/slots): clarify slot presence phrasing * docs(guide/components/slots): clarify slot presence phrasing The original phrasing "whether or not a slot is present" was potentially misleading, as it suggested checking the existence of the slot itself rather than whether content was passed to it. Updated the explanation to clarify this distinction for better readability and accuracy. * docs(guide/components/slots): clarify slot presence phrasing (2) The phrase "When the header/footer/default is present..." could be interpreted as referring to the slot's definition, rather than whether content is provided for the slot (#95)#96