document make -s CFLAGS='-g -O'

[積丹尼 Dan Jacobson]

(info "(make) Overriding") says
  Each time you run 'make', you can override this value if you wish.
  For example, if you say 'make CFLAGS='-g -O'', each C compilation
  will...

Wouldn't "make CFLAGS='-g -O'" be clearer?

Also add an example of if one should write

make -s CFLAGS='-g -O'

or

make CFLAGS='-g -O' -s

when combining with e.g., -s.