eldoc-echo-area-use-multiline-p

eldoc-echo-area-use-multiline-p is a variable defined in `eldoc.el'.
Its value is

truncate-sym-name-if-fit

Documentation:
Allow long ElDoc messages to resize echo area display.
If value is t, never attempt to truncate messages; complete symbol name
and function arglist or 1-line variable documentation will be displayed
even if echo area must be resized to fit.

If value is any non-nil value other than t, symbol name may be truncated
if it will enable the function arglist or documentation string to fit on a
single line without resizing window. Otherwise, behavior is just like
former case.

If value is nil, messages are always truncated to fit in a single line of
display in the echo area. Function or variable symbol name may be
truncated to make more of the arglist or documentation string visible.

Note that this variable has no effect, unless
`eldoc-documentation-function' handles it explicitly.

You can customize this variable.