manual: add section about depending on target and toolchain options

Currently, the comments in Config.in files when depending on toolchain
options are not at all lined up.  This patch adds a section to the
documentation that explains which format is to be used.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>