The current documentation reads:
"If SYMBOL is non-nil, store the menu keymap in the value of symbol,
and define symbol as a function to pop up the menu, with doc as its
doc string."
Lennart suggested:
"SYMBOL should be a symbol. If it is not the symbol nil ..."
Obviously "If it is not the symbol nil" is the same as "If it is non-nil",
so we have the equivalent of "If SYMBOL (which should be a symbol) is
non-nil, ...". I fail to see the point in merely adding that an argument
named SYMBOL should be, um, a symbol.