config.5: Document CONF_CFLAGS under 'makeoptions'

This variable serves to tweak CFLAGS from the kernel configuration file,
easing some custom kernel builds. It was introduced by bde@ in 1999.

The description has been kept simple on purpose. It does not completely
reflect reality as the build infrastructure actually appends variables
to CFLAGS after CONF_CFLAGS has been processed in a few cases. However,
these cases do not collide with expected common uses of CONF_CFLAGS, so
have not been mentioned.

Reviewed by: markj (older version)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50544