More involved/explicit version of D22449.

These tools don't deserve mentioning in the handbook at all. When considering the audiences of the handbook, particularly those less experienced in building their own ports and troubleshooting build failures, we should not be encouraging having users shoot themselves in the foot. While building in unclean environments can prove useful, said usefulness is limited to those who intrinsically know what they are doing, and mentioning such doesn't provide value to handbook audiences. Ports-related mailing lists and other communication mediums see endless amounts of reports that to trained eyes are pilot errors, all of which can be avoided by encouraging building under clean environments. Further, certain ports effectively (and intentionally) require clean build environments due to conflicts.