Help: Clarify/make consistent the APPLE, UNIX and WIN32 variable docs

The first line of the wording of the WIN32 and UNIX variable docs has the potential to mislead the reader into thinking the variables provide details of the host platform rather than the target platform. For both of these variables, the rest of the (short) description makes it clear, but someone reading in a hurry could gloss over the rest. Better to just make them both clear from the outset.

The docs for the APPLE variable were more ambiguous, potentially implying host platform details rather than target platform. Furthermore, the docs had not been updated to mention Apple platforms other than OSX.