manual: add explanations on limitations about using O=...

Using a relative path for O=... has limitations, since it is interpreted
relative to the Buildroot tree, and thus may lead to unexpected results.

For example, running this:
    make -C buildroot O=my-O

will not create my-O in the current working directory, but as a
sub-directory of the Buildroot tree, here in buildroot/my-O

Explain this in the manual (as is similarly done for BR2_EXTERNAL).
Also add a note that $(O) will be created if missing.

Also change O=.. and -C .. to O=<...> and -C <...> to make it explicit
this is an ellipse, not a relative path.

Reported-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>