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>

docs/manual/adding-packages-directory.txt

@@ -222,8 +222,8 @@ use in the comment.
 * Kernel headers
 ** Dependency symbol: +BR2_TOOLCHAIN_HEADERS_AT_LEAST_X_Y+, (replace
    +X_Y+ with the proper version, see +toolchain/toolchain-common.in+)
-** Comment string: +headers >= X.Y+ (replace +X.Y+ with the
-   proper version)
+** Comment string: +headers >= X.Y+ and/or `headers <= X.Y` (replace
+   +X.Y+ with the proper version)
 
 * C library
 ** Dependency symbol: +BR2_TOOLCHAIN_USES_GLIBC+,