doc: environment: clarify env precedence

Since commit 5cf6a06a it is possible to have both text-based and
old-style C environment files. But so far the environment documentation
has not reflected this change. This commit fixes that.

Signed-off-by: Ricardo Simoes <ricardo.simoes@pt.bosch.com>
Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>