doc: Correct the return type of float comparison

doc: Correct the return type of float comparison

Documentation for `__cmpsf2` and similar functions currently indicate a
return type of `int`. This is not correct however; the `libgcc`
functions return `CMPtype`, the size of which is determined by the
`libgcc_cmp_return` mode.

Update documentation to use `CMPtype` and indicate that this is
target-dependent, also mentioning the usual modes.

Reported-by: beetrees <b@beetr.ee>
Fixes: https://github.com/rust-lang/compiler-builtins/issues/919#issuecomment-2905347318
Signed-off-by: Trevor Gross <tmgross@umich.edu>
	* doc/libgcc.texi (Comparison functions): Document functions as
	returning CMPtype.