docs/manual: enhance doc for BR2_TOOLCHAIN_HEADERS_AT_LEAST_X_Y

The BR2_TOOLCHAIN_HEADERS_AT_LEAST_X_Y symbols can be used to denote the
lowest required kernel headers version, but also the highest supported
version. Document that.

Use the `...` construct instead of +...+ used eveywhere else, otherwise
asciidoc would render +<=+ to the ⇐ arrow, while `...` does no rendering
at all.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>